Dura Ace 7700 rear derailleur will not fit into hanger
Practically all derailleur hanger bolts are M10x1.0mm, so it's strange that the derailleur bolt will not screw into the hanger.

Possible reasons:


The thread in the hanger is damaged
The bolt thread is damaged (but from the pic it looks OK)
It's a hanger for a Campagnolo derailleur which uses M10x25TPI - but that's very close to M10x1.0mm, you should at least be able to start the bolt in the thread.


Do you have another bike to try the derailleur bolt in?  Perhaps obtain a M10x1.0mm bolt from a hardware store and see if it fits the hanger.

Googling for 'replacement derailleur hanger' should get you some results. Currently Wheels Manufacturing offers a wide range of replacement hangers and provides a search by bike brand. Start here.