It's a fundamental problem of open source projects hosting on GitHub. I don't think the the Fossil colocation model is necessarily right, but some open protocol is needed so that issues can be easily migrated between these services.
Back then, I managed to get approval from all our contributors/issue authors so that their individual bugs and comments would show up as their individual bugs and comments, as well as retaining issue numbers.
This is the code for it, if anyone is interested, but as I said the whole thing is unsupported now: https://github.com/jleclanche/bugzilla-to-github
https://gist.github.com/jonmagic/5282384165e0f86ef105
But it doesn't let you change ownership of issues or comments; everything appears as created by the importing user.
Also, you can impersonate commits freely.
I don't think allowing full import would be a big deal, especially if the UI made it clear that the comments were imported.