No, I don't think you can get FaceGen to work on any figure. It generates its own meshes, it doesn't modify existing ones.

The way Facegen users get their heads on V3's body is to edit it on, the way people frankenstein Vicky's head onto Posette's body. Or they fix it in postwork. This is why I think the best solution would be for DAZ to make or license their own plugin. One that works on unimesh heads.