Set the camera to FS7, your Rec Gamma and Gamut, then output Gamma and Gamut. Name your LUT, set the LUT type to FCPX and generate.
I don't have any FS7 footage to test it with, but below is an FS7 S-Log S-Gamut3.cine to Rec709 LUT I generated with it (3D 32x32x32 LUT). I also don't have a way to test it in FCPX right now so it may not work, or it may look awful. If you test it, let me know if it works.