<< but I don't yet know how to add another .obj file into it.

Just "import" another 3D file(s) of any nature(s) of format(s)!
and this any times you want! :)

Just carreful of the position of Import as there are from position to origine!
But not really a problem as you can Move Rotate Size etc... them as you want!

A very very funny easy program and free with some few limitations ;)

In French : site and Prog! ;)
http://simlabfr.weebly.com/light.html