@Lione
You never asked me that question, didn't you?
-In FaceGen you can select any skin texture during the creation process, you have locally on your HDD/SSD. The face and skin (!) will be automatically matched.
If you always save the face in the internal FaceGen formate, too, you can create it with different skin textures, later.
An other way is to match a FaceGen face in Photoshop (or any other tool that uses layers) to a given skin:
-Load the original face texture in a first Photoshop layer.
-Load the FaceGen face texture in a second layer, lying over the first one.
-Erase the seams of the FaceGen texture in a way, that only the pure face remains, and the original face textures seams are shining through. Erase the neck region, the ears region, the forehead region, aso. Literally go around the whole face with the eraser. Use a big (~1000) and soft erase tool, so the edges are blurred into the underlying original face texture.
-Use the very basic color tools on the FaceGen face (ctrl + U) to somewhat match saturation, color and brightness to the underlying original face.
-Merge it, Save it, Ready.
If you are used to it, this is done in maybe 3-5 minutes or less. The trick is to not touch the original skin seams, because matching seams is a pain in the ass.
If you are a somewhat talented Photoshop user, you most probably want to fix some of the FaceGen texture's issues during this process. Erase the ugly lips, for instance, to use the lips of the underlying original texture, ... or mix and match some eyebrows from an other face texture, aso. Depending on your own taste and standards, this can take quite some time.
You can create a high number of new faces by mixing the best parts of some custom faces together in different ways.
I do this all the time with my own FaceGen faces, because I always use some few custom body textures, and I want to match the many faces to the few good skin(s), not the other way around. Therefore, I just load a good custom skin preset of mine, and can simply browse through a high number of compatible FaceGen faces.
Hope this may help a bit.
PS: For doing this, you will surely need to have the skin textures. You can't do this with the internal protected game textures (if you can't get your hands on that textures somehow).