Edit:
WWW Link for the file. Keep in mind that if you have "Neutral colinies" maxed on the map designer (or in galixy forge how ever that works) there will be milita planets just no combat ships in orbit.
it is possible to do if you modify a few files wich determine the milita spawn for a planet. i think it was the GalaxyScenarioDef.galaxyScenarioDef, download the mod tools, and then find the game reference data, and open the file stated above, around half way down is the following. if im correct this is what you would need to modify (i personally haven't tried this). hope it helps.
PlanetItemsTemplate
templateName "Template:LocalMilitiaVeryWeak"
subTemplates 0
groups 2
group
condition
type "PlanetOwnerIsMilitia"
param ""
owner "PlanetOwner"
colonizeChance 0
items 6
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:Light
item "Tech:Module:CrystalExtractor"
item "Tech:Module:MetalExtractor"
item "Tech:Module:GaussDefense"
group
condition
type "NoPlanetOwner"
param ""
owner "RandomMilitia"
colonizeChance 0
items 2
item "Tech:Frigate:Light"
item "Tech:Frigate:Siege"
planetItemsTemplate
templateName "Template:LocalMilitiaVeryWeakUncolonizable"
subTemplates 0
groups 1
group
condition
type "Always"
param ""
owner "Pirate"
colonizeChance 1
items 2
item "Pirate:Frigate:Scout"
item "Pirate:Frigate:AntiFighter"
planetItemsTemplate
templateName "Template:LocalMilitiaWeak"
subTemplates 0
groups 5
group
condition
type "PlanetOwnerIsMilitia"
param ""
owner "PlanetOwner"
colonizeChance 0
items 6
item "Tech:Frigate:Light"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Siege"
item "Tech:Module:GuassDefense"
item "Tech:Module:MetalExtractor"
item "Tech:Module:CrystalExtractor"
group
condition
type "PlanetOwnerIsMilitiaWithChance"
param ".5"
owner "PlanetOwner"
colonizeChance 0
items 4
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:AntiFighter"
group
condition
type "PlanetOwnerIsMilitiaWithChance"
param ".5"
owner "PlanetOwner"
colonizeChance 0
items 3
item "Tech:Module:GuassDefense"
item "Tech:Module:GuassDefense"
item "Tech:Module:GuassDefense"
group
condition
type "NoPlanetOwner"
param ""
owner "RandomMilitia"
colonizeChance 0
items 3
item "Tech:Frigate:Light"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Siege"
group
condition
type "NoPlanetOwnerWithChance"
param ".5"
owner "RandomMilitia"
colonizeChance 0
items 2
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
planetItemsTemplate
templateName "Template:LocalMilitiaMedium"
subTemplates 0
groups 6
group
condition
type "PlanetOwnerIsMilitia"
param ""
owner "PlanetOwner"
colonizeChance 0
items 10
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Siege"
item "Tech:Module:GaussDefense"
item "Tech:Module:GaussDefense"
item "Tech:Module:GaussDefense"
item "Tech:Module:HangarDefense"
item "Tech:Module:CrystalExtractor"
item "Tech:Module:MetalExtractor"
group
condition
type "PlanetOwnerIsMilitiaWithChance"
param ".5"
owner "PlanetOwner"
colonizeChance 0
items 5
item "Tech:Frigate:LongRange"
item "Tech:Frigate:LongRange"
item "Tech:Frigate:AntiFighter"
item "Tech:Module:GaussDefense"
item "Tech:Module:GaussDefense"
group
condition
type "PlanetOwnerIsMilitiaWithChance"
param ".5"
owner "PlanetOwner"
colonizeChance 0
items 4
item "Tech:Frigate:Siege"
item "Tech:Frigate:Siege"
item "Tech:Frigate:AntiFighter"
item "Tech:Module:HangarDefense"
group
condition
type "NoPlanetOwner"
param ""
owner "RandomMilitia"
colonizeChance 0
items 4
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Siege"
group
condition
type "NoPlanetOwnerWithChance"
param ".5"
owner "RandomMilitia"
colonizeChance 0
items 3
item "Tech:Frigate:LongRange"
item "Tech:Frigate:LongRange"
item "Tech:Frigate:AntiFighter"
group
condition
type "NoPlanetOwnerWithChance"
param ".5"
owner "RandomMilitia"
colonizeChance 0
items 3
item "Tech:Frigate:Siege"
item "Tech:Frigate:Siege"
item "Tech:Frigate:AntiFighter"
planetItemsTemplate
templateName "Template:LocalMilitiaStrong"
subTemplates 0
groups 10
group
condition
type "PlanetOwnerIsMilitia"
param ""
owner "PlanetOwner"
colonizeChance 0
items 10
item "Tech:Frigate:Light"
item "Tech:Frigate:LongRange"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Heavy"
item "Tech:Frigate:Heavy"
item "Tech:Module:GaussDefense"
item "Tech:Module:GaussDefense"
item "Tech:Module:HangarDefense"
item "Tech:Module:MetalExtractor"
item "Tech:Module:MetalExtractor"
group
condition
type "PlanetOwnerIsMilitiaWithChance"
param "0.3"
owner "PlanetOwner"
colonizeChance 0
items 5
item "Tech:Frigate:LongRange"
item "Tech:Frigate:LongRange"
item "Tech:Frigate:Heavy"
item "Tech:Module:GaussDefense"
item "Tech:Module:HangarDefense"
group
condition
type "PlanetOwnerIsMilitiaWithChance"
param "0.3"
owner "PlanetOwner"
colonizeChance 0
items 8
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:Heavy"
item "Tech:Module:HangarDefense"
group
condition
type "PlanetOwnerIsMilitiaWithChance"
param "0.3"
owner "PlanetOwner"
colonizeChance 0
items 5
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Heavy"
item "Tech:Module:GaussDefense"
group
condition
type "PlanetOwnerIsMilitiaWithChance"
param "0.3"
owner "PlanetOwner"
colonizeChance 0
items 5
item "Tech:Frigate:Siege"
item "Tech:Frigate:Siege"
item "Tech:Frigate:Heavy"
item "Tech:Frigate:Heavy"
item "Tech:Module:HangarDefense"
group
condition
type "NoPlanetOwner"
param ""
owner "RandomMilitia"
colonizeChance 0
items 5
item "Tech:Frigate:Light"
item "Tech:Frigate:LongRange"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Heavy"
item "Tech:Frigate:Siege"
group
condition
type "NoPlanetOwnerWithChance"
param "0.3"
owner "RandomMilitia"
colonizeChance 0
items 3
item "Tech:Frigate:Light"
item "Tech:Frigate:LongRange"
item "Tech:Frigate:LongRange"
group
condition
type "NoPlanetOwnerWithChance"
param "0.3"
owner "RandomMilitia"
colonizeChance 0
items 5
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:Light"
item "Tech:Frigate:Siege"
item "Tech:Frigate:Siege"
group
condition
type "NoPlanetOwnerWithChance"
param "0.3"
owner "RandomMilitia"
colonizeChance 0
items 4
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Light"
item "Tech:Frigate:Heavy"
group
condition
type "NoPlanetOwnerWithChance"
param "0.3"
owner "RandomMilitia"
colonizeChance 0
items 4
item "Tech:Frigate:AntiFighter"
item "Tech:Frigate:Light"
item "Tech:Frigate:Siege"
item "Tech:Frigate:Heavy"