Welcome, first of all. If you are new to TG, you're aiming high
Let me tell you what I can tell you; you can't really create assets in TG, unless you mean things like rocks (you can procedurally mould some stuff from displacable cubes). But nothing like huts or people.
You can import them very well though; best is an obj file. You may have to reassign some textures, so check that out after importing. TG works in meters, so try to get your obj's in meters (Poseray is a handy in between). You can save these assets as (native) tgo, which can be loaded in subsequent sessions easily, including all added procedural nodes.
Having playable characters would need a different app, IMO.
Exporting terrain is possible with the micro exporter, but I have no experience with that.
Hope this helps.