You should be able to send it directly to FCPX using the "Send To Final Cut Pro" menu item. Per the documentation, they say:
Do one of the following:
- Select the project in the browser, or double-click the project to open it.
With the project open, click anywhere in the timeline.
Note: If you can’t find the project you’re looking for, verify that the correct library is selected.
- Choose File > Send Movie To Final Cut Pro.
Final Cut Pro X opens. A new library appears in Final Cut Pro X with the name of your iMovie library, and a new event within that library contains the movie project you sent.