iMovie and FCE's plugins are not compatible with each other, so that would explain why your iMovie effects aren't transferring over in a direct export.
One way to do what you want to do is to the "Export Using Quicktime" option in iMovie to output a self-contained, full-res Quicktime file (which would render the video with your effects intact) and bring that file into FCE. It's sort of a crude process, but it would nonetheless work.
If you're using iMovie solely for its stabilization effect (due to its strange absence in FCE), there are several plugins available for FCE that you give you that functionality.