If your iMovie event is DV, the easiest way is to go to FCPX and use FILE/IMPORT... IMOVIE EVENTS
This will give you access to your DV without doubling up on space. The iMovie Event and the FCP Event will be distinct but will point to the same clips. Only the thumbnails, cache and analysis files would be different.
If that works for you, great.
If you still are interested in the ElGato solution, I have a different ElGato product (The EyeTV Hybrid, with EyeTV3 software,) but I get really good results through it. The ElGato Video Capture product may be feature constrained relative to the EyeTV hybrid, but I don't know for sure since I dont have one.
I am actually recording a TV show with the ElGato right now so I can't get down into the menus right now, so this is from memory, but as I recall - as I capture, I can use a very high quality mode that gets captured as MPEG2, or I can select a more compressed format that is h.264. For highest quality, the MPEG2 seems to work best. For exporting, choose QuickTime and then choose either ProRes 422 or Apple Intermediate Codec or DV. Personally I like ProRes 422 for FCP and AIC for iMovie. In my experience, DV is about 20% bigger than AIC with no visible difference in quality.