So the devs were kind enough to reply in the KS comments when I posted the same question there. The response is as follows....
Yes - we're going to allow you to import your own artwork & sound. We don't have any DLC plans for the moment but al the content that we provide can be used in your own campaigns. The editor will also support integration with Steam workshop btw.
If we do release new content for the editor, it will not be a DLC.
About importing models, objects and textures: we are looking into it, but yes, that's the idea. Wouldn't want it any other way.
Sweetness and light!
