We are attempting similar job for our charity in Oxford. We are importing video shot in NTSC using a PAL device that provides some NTSC playback capability.
Even with Final Cut Pro with a lot of setting options, it sems a bit tricky, and a little unreliable, so I have doubts of your success with iMovie.
We have had to try various import & device controller combinations. Also, apparently the firewire bus sets itself for either a PAL or NTSC video signal and requires the computer to be restarted in between. So there are various issues at play.
As you know, the colour space, the frame size and the frame rate are all different between the two standards. And I think NTSC playback capability from a PAL device doesn't actually perform a real conversion, just enables it to be viewed.
So try to see if there is an iMovie setting that allows you to switch input stream to NTSC. And then worry about conversion later, using QuickTime or something.
We have some success using NTSC import settings and setting device controller to non-controllable device. We are now working on getting the best conversion from one to standard to the other within the computer.
Good luck.
Moxham