cam i use .obj files with Motion

No...

What app are you using to create the .obj file? If your using Blender you can export to the Animation (with alpha channel). Bring that into Motion. (Set the anti-aliasing to 16 and check the full sample box. Go and have a few cup of coffee.)

I have script that allows you to export a Blender file to Motion. I haven't figure out how to use it. (I need to ask that question in here.)

Blender and Motion 5:

http://youtu.be/HgDiInbUDp8

You can bring a .DAE file into FCP X with the "3D Supercharger V2" plugin, which will soon be coming to Motion, with major updates in a month or two, from what I'm told.

http://www.iloveqc.org/marketplace-home/final-cut/3d-supercharge-v2-detail

It is all done through Quartz Composer.