you need to create a new Research subject. Doing so is harder than it sounds, as you need an open space in the trees, to assess how many labs (the tier #) you need to access it, if it's still accesible w/out said # of labs (most, if not all researchable ship types aren't), the cost/time of the research, etc. etc.

You could do this by tying it with the Cannon research for each race which is on the 8th tier. All you would need to do for each races capitalship entity file is look for the section which says

Prerequisites
    NumResearchPrerequisites 0

and add this for Tech

Prerequisites
    NumResearchPrerequisites 1
    ResearchPrerequisite
        Subject "RESEARCHSUBJECT_PLANETMODULEACCESS_CANNONTECH"
        Level 1

replace the bold lettering with

RESEARCHSUBJECT_PLANETMODULEACCESS_CANNONPSI for Advent Capitalship

RESEARCHSUBJECT_PLANETMODULEACCESS_CANNONPHASE for Vasari Capitalship

in their respective entity files. Job done.