Third Party Compendiums

Can compendiums for 3rd party products be created if you have the permission of the publisher?

Hi Nicola! With some caveats, yes. Many products on Roll20 have their own independently-produced compendiums. You will need communication between the publisher and set up an official working relationship with them and Roll20, whose assistance you will need to create the compendium. This is speaking in terms of a commercial, Marketplace product. Private compendiums are still not a thing.