I'm unsure because the "Send movie to Final Cut Pro" option is missing from the File menu whatever project I try to open.
Is there something I need to do to have this option?
Well according to Apple you need to upgrade to Final Cut Pro X. Just US$ 300 bucks. More info here: https://help.apple.com/imovie/mac/10.1/#/movcbf7e2a3f
A cheaper alternative would be exporting a XML and use FCP 7 to import the file and then reconnect the media you used on iMovie.
Last case scenario: do most of the work over iMovie and export a big quicktime clip to open in FCP 7 and finish your project.