You can export out your clip as ProRess 4444 or Animation. I prefer Animation. To do this mark an In and Out point of your keyed clip. Make sure your clip is NOT rendered. If it is rendered you will end up with a black back ground and not a transparency. Then go up to File, Export, Using Quicktime Conversion. Make sure Format is set to QucktTime Movie. Click on the Options button. In Video click on the Settings button. Change Compression Type to Animation. Change Depth to Millions of Colors +. Then import the clip into Motion and you are good to go.