Q2A pages are file-based -- all .php files. MODX stores pages in the database and doesn't allow PHP code in the pages.
In order to fully integrate it with MODX, I think you'd have to turn all these files into snippets and modify them to run inside MODX: https://github.com/q2a/question2answer/tree/master/qa-include
I think you could install it in a subdirectory as they describe and just provide a link to its index.php file in MODX. You could style it to look like your MODX site, but if you want to synchronize users between MODX and Q2A, you'd have to write some custom code.
[ed. note: BobRay last edited this post 4 years, 4 months ago.]