Assuming you know to edit the entity manifest for any new entity files.
Adding a new research file is as simple as creating the file, editing it's position, costs and buffs and then adding it to the appropriate PlayerXXX.entity file and incrementing it's research count. I believe the dev.exe will warn if you try to overlay an existing research. A good way to avoid this is have the game running with the research tree displayed so you can check if the position is free.
researchInfo
count 226
entityDefName "RESEARCHSUBJECT_CAPITALSHIPACCESS_PLAGUE_COLONY"
entityDefName "RESEARCHSUBJECT_RESEARCHACCESS_PLAGUE_CMD_T0_01"
entityDefName "RESEARCHSUBJECT_RESEARCHACCESS_PLAGUE_CMD_T0_02"
...