You’ll need to setup SSO. Note that it’s possible to do that between XWiki and Discourse (this is what we have setup on forum.xwiki.org). Search for that on this forum and you should find some config for it.
So with some custom skin, you can replace the XWiki comments area with Discourse threads.
FTR I did something similar years ago on my blog but with IntenseDebate, see http://massol.myxwiki.org/xwiki/bin/view/Blog/AnonymousComments (it might be a bit old now but the strategy should still work).