CTRL+click on the alpha to select your bg area.
Select your alpha, and go to SELECT > LOAD SELECTION.
Iether way, you should now have a selection that you can use to place your bg into.
NOTE: Depending on your alpha preferences from another app, you might have to iether INVERT your selection, or INVERT your alpha to get the right selection
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS