Mark an in and out point on the time line of the keyed footage in FCP. Make sure the clip is not rendered. Rendering will make it so it generates a black background of your keyed footage when you export it. Then go up to File, Export, Using QuickTime conversion... Make sure the Format is QuickTime Movie. Click on the Options Button. This will open up a Movie Settings menu. Click on Settings. Choose Animation for the Compression Type. Make sure the Depth is set to Millions of Colors +. The + is the alpha channel. Click on OK. Then click on OK in the Movie Settings window. Choose a location and then export out the clip. Import the clip into Motion and you have a keyed clip with the alpha channel. Hope this helps and best of luck.