You kind of can do it with Poser. It's not something ordinary users do, but neither is geografting.

There's things like Little Dragon's Mia, with the Millennium Cat's head and cat feet on Vicky. And there have been centaurs, with, say, M4's upper body replacing the Milhorse's head and neck. And a lot of the things people are doing with geografting were done in Poser using conformers: wings, tails, etc. RDNA had a multiarmed Kali figure based on V4, though I don't recall how it was made.

The problem with these things are textures, but as I understand it, this is an issue with geografting as well.