Unity Runtime OBJ-Export of specific game objects

Hi, I have Gamobjects (see Picture below) I want to export into a OBJ on runtime. The Colours should be preserved. The big part is an OBJ Import and the small spheres are regular gamobjects. User should be able to press a button, the native file browser (OSX/Win) should appear and the user picks a path and filename. Then save the gameobjects there in a single obj.