I think you need to first export it to QuickTime from iMovie (either as "Full Quality, Large" or "for iDVD"--they're the same settings), then go into QuickTime Pro and export that movie as a DV stream.
Let me know if that doesn't work, it's off the top of my head as I haven't done it in a while.
Alternatively, you could export it from iMovie to DV tape and then capture it in FCP.
But note there is no way to save and edit your titles, transitions, etc.
MM