if I understand correctly, you should use an external 3d program to manually define UVs on your model and map the hand images onto these. If you want Photoscan to do the work for you, you need to reconstruct the model using photos from your original
hand.
You may try the following trick: take as many photos of your hand as you have poses from the gypsum model you used, and as close as possible in size and orientation. Give these photos the same filenames of the ones of the gypsum hand, then open a new chunk, import there the new photos, import the camera alignments from the reconstructed gypsum hand (you should save it first after the reconstruction), import the mesh from the gypsum hand.
At this point your new chunk has the reconstructed mesh, the reconstructed camera alignment and the new photos and you can create the texture.
It will most likely come out horrible and useless, unless you are *very* accurate in photographing your hand *exactly* like the fake one was photographed.