How to add Agnes Crumplebottom to the family tree?

It is, but you'll need to use the cheat that allows you to completely edit your sims in CAS. By doing this in World edit mode, meaning not actually playing with the sims, it should preserve what the game has already set for them. To play it safe, before doing this make a separate save of your game. This was if something goes wonky, then you won't lose what progress you currently have.