Like letting them build simple levels with a very simple level editor that allows them to put around only objects from the game itself and assign starting points for players.
could always put your own level editing system on top, kinda like halo did with forge
There is a long running thread on topic here:
Basically you would need to build your own mesh import system (you can not use any of the code from the source to do ), which means implementing the FBX SDK on your own, quite an undertaking. That is only meshes though, you need to be able to import it all. Then you need your own custom editor, scripting system etc etc.
Have a look through the thread, Sweeney responded laying out the requirements. It can be done but would be simpler to have them pay $20 once and have access to the engine. 
Thanks. The game is going to be free so i will take a look at . also, i don’t want people to import anything, want them to use only meshes that are already existed in the game.