Shiroi Koumori
Oh my! That is troublesome. If you do not intend to move the fingers in the future, use superglue to stick them in place permanently. However, if you intend to move them in the future, use a little bit of modeling clay and mold it to the shape of the finger joint, let it dry first, trimming might be needed if you molded it a little bit bigger than the hole, then put it to the hand.