I wrote an article in the thread of the doll of "Lamia of the Dark":
http://www.dolliehsanctuary.com/sanctuary/viewtopic.php?f=6&t=6286
Volks hand joint is about 4mm diameter, and Obitsu is 3mm.
I think your easy way is:
1) Wrap Obitsu joint with tissue paper.
2) Glue it together (drop instant glue... bit smell).
I tried it some days ago.
I wish this experiment solve the problem of Lamia (or someone).
1) Cut off the shaft of Obitsu hand to fit Volks wrist socket.
2) Cut tissue paper to same size as shortened wrist shaft.
3) Coil the paper to joint 2 or 3 times.
4) Drop instant glue and wait some minutes.
5) After the glue dried, connect the hand and arm, and check friction. If you feel it is loose, drop more glue. Or if it is tight, sand the shaft simply.
BTW, This modifying "cannot return".
If you don't have extra spare hand for your Obitsu, I don't recommend this method.