The problem is that I can't marry them OR get them to add to active family
Huh. In my game, I had a sim befriend a child ghost, move her onto the lot, bring her back as a playable, and then marry her off to his son. They now have a ghost baby.
I'd ask how you're not able to do this stuff, but if you knew that, the question might be unnecessary.
1) Went to graveyard
2) befriend a ghost
3) invite him / her to my home
4) Gave him / her ambrosia
5) Became human again
6) invited the same person back to my home
7) Tried to "add to active family" = nothing
person became my bf/gf = but add to active family doesn't work...