I'm not sure that I completely understand what you're asking, but what I've done with 16:9 source is, within FCP, double click on the clip and go into the Motion tab. There, click on Distort. Enter the values so they look like this:
-360, -180
360, -180
360, 180
-360, 180

Save as a motion favorite, and apply to anything that's 16:9. If I'm doing this for a whole vid, I've just edited it all in squishyvision, then exported the sequence, reimported it as a single clip, and applied the Distort at the end of the process.