Yikes...
I would have thought that's a pretty extensive change...
But one thing you could try in a completely isolated test board is removing the unique index on the username field with this query:
Code:
ALTER TABLE `xf_user`
DROP INDEX `username`
I haven't even begun to think about the problems this could cause, though, so please do proceed with caution.
There's quite a few cases, I think, where XenForo assumes the username is unique so I think this could require some pretty extensive development. I know why you want to do it though... Just a big task, I think.