I read about Solasta supporting customer made maps and campaigns. I played customer made campaigns in other games in the past and I'm wondering how Solasta handles that: can new campaigns be integrated into the main game so they appear to be something similar like side quests, or are they always started independently from the main game?

It's probably possible to "hack" the main campaign, but the expected way to do user-generated content is as separate campaigns, which you choose when you start a new game. I have not looked on Steam, but there are no Nexus mods I know of that alter or extend the main campaign (except the one that skips the tutorial, which was obsoleted by the 1.2 update).

Thanks for the info! I think it would be a cool thing if they'd allow to extend the main campaign, but i understand if it's not the way the developers intended it.