I currently have site running xenforo, wordpress, and invision power board,
Does the importer support the latter two? is it possible for me to use importer to merge all data?
We do have an importer for Invision forums (currently only for XF1), importing things like users, forums, threads, posts (and various other elemtns). We don't have an importer for WordPress though.
No, that is something that's done separately. If you use the importer option to maintain IDs, this can generally be done directly at the web server level (.htaccess or similar).