FBX Import question

Hi all!

Can anyone tell me where can I find in the 4.1 source the functions to import outer fbx files into a package?
What I want to achieve is giving the ability to users to modify the game. So they can create 3d meshes and upload it to the game. The game will create a new empty package every day, and the uploaded fbx will be put in there. On the next day this package will be closed, and downloaded by all of my clients who can start to use it. For example think of a racing game where initially you have 8 cars, but as time goes by, users start to modify things and they can choose from hundreds of cars after a few months.

Thanks in advance: