You can move your forum to a subdomain and keep it in WordPress e.g. https://forum.mydomain.com
If you do that make sure you add the redirects in the HubSpot COS to point mydomain.com/forum to forum.mydomain.com to make sure your users (and Google) don’t get lost.
As far as I know (someone please correct me if I’m wrong) HubSpot COS can’t handle a redirect as part of a URL i.e. mydomian.com/forum cannot be directed to another site without the URL changing.
Also, depending on how geeky you are, you can use WordPress Hooks from the Forum to push or pull data into HubSpot with the API (yes, I need to get out more).