Join a research group at a university I do not attend
This will depend on the field. In my area (in computer science), research groups frequently work with external collaborators from industry. From the perspective of a research group, such collaborations can be very desirable, since practical motivations or, even better, applications can improve their research. There are even conferences with a specialized "industry" track, where perspectives of practitioners are sought explicitly.

Teaching-wise, this will depend on the country as well. In Germany, it's not unheard of that people from industry are employed on a part-time contract to give a lecture with a practical focus.

I think you should directly contact a suitable research group (professors and maybe post-docs), rather than the university, since employees of the university administration might be unaware of those specifics.