Stuff from other games cannot legally be added to FreeSO.
Custom content is ok. (with permission)
And there is good stuff included in the TSO download that doesn't appear in the game without help. Some of that will simply need to be added to the buy and build mode catalogs. Other stuff needs additional work to actually function in the game. You'll find that sort of thing in Buildmode's Terrain tool for now.
Some of us have already started working on some of these items. I'm not sure how familiar you are with "sims hacking" but the more experiences you have with Simantics, the easier it'll be for you to update objects. It helps you to recognize when TS1 and TSO are using the same number for a primitive or global call, for example.