I'm not sure if there is a option in imovie to help you convert files to different formats. Umm, the way that I found the free conversion software that I use (sorry its for a windows machine) is to just try different ones. Read about them, see if its got all of the things that you want it to have, Download and try.
It is risky doing things this way, because of viruses and whatever else. I can't think of any free conversion software for macs but I will have alook and post back here if I find anything.