If you create a blueprint package calling the tag 'remnant', which should pull all the ships and high-end weaponry that outfits Remnant ships, that'll 'make them craftable' although the UNBOARDABLE tag will make it impossible to recover them if destroyed.
If you want to go further and make a more user-friendly mod, you can create a set of ship skins that remove the UNBOARDABLE tag, or have some minor visual alterations, and tag them with something like 'remnant_playable' and have your blueprint package call that tag. Then you'll have a set of customized Remnant ships that don't need to replace any vanilla content. You could repaint them to your heart's content, change slot types, and generally monkey around with the hulls to have custom AI-controlled defense ships.
Could you add it to a running game? mmm... probably, although IIRC rare blueprint packs are scattered around the sector at sector generation, so they wouldn't be included. You can solve that with Console Commands, of course.