That will require conversion to a format that iMovie can use, which will give slight quality loss.
You forgot to mention what Mac you have, what verisons of OS X and iMovie, so:
iMovie 11 - cameras supported:
iMovie 9 - cameras supported:
iMovie 8 Camcorders supported:
Digital camera RAW formats supported by OS X Lion:
Digital camera RAW formats supported by OS X Snow Leopard:
iMovie 11 handles h.264 well. H.264 is a highly compressed format. I would advise that when you import, you "Optimize" the video. This will convert it into Apple Intermediate Codec for editing. After editing, iMovie will convert it back to h.264 for playback on the devices of your choice.
For why AVCHD and h.264 should be converted to Apple Intermediate Codec, see my post here. AVCHD and h.264 are similar long GOP codecs. iMovie will always convert AVCHD to Apple Intermediate Codec. With h.264 you have a choice, because some h.264 cameras shoot standard def sizes, and you can probably edit them natively without conversion.
Owing a new Nikon D810 but met the similar trouble as you when editing MOV files in iMovie. Search high and low, I got know iMovie can’t handle the MOV files encoded in H.264, a heavy compressed format. To solve the problem, I have tried to convert D810 MOV/H.264 to iMovie format- Apple InterMediate Codec on Mac OS X. Yes, it helped me out. I think it would also be a solution for you.