The dokuwiki page tips and tricks
https://www.dokuwiki.org/tips#forum_systems_only lists a couple of instructions for integrating a forum on a dokuwiki site. I have integrated phorum (
http://www.phorum.org/) on a site by following their forum on integrating with a CMS.
Depends on what forum software you are interested in using. Most should integrate okay so you may want to pick a couple of forum software products and then see what is available to help integrate them.
Regards
Tom