Yep, just make sure you drop a 1080 file on the timeline first so your project is in 1080. Then, whenever you add another 720p clip, it'll set the zoom to 150 in the motion tab for you.
mmm so in theory one could mix over-under cranked 720PN variable frame rate footage onto the same time line as 1080, FCP will auto up rez it, (as long as your sequence settings are set to 1080) if not just enlarge the 720PN footage with the zoom motion???
If this is possible would one run into any other problems along the way..I don't know, in the export stage say??