The work is actually done in Sonicfire Pro. With the plugin, you lay out the generator to the time you need then click Generate Music in the Inspector. This opens Soncfire where you select and work on your music. When ready, click the “Send” button to import your customized soundtrack into FCPX as a new Event. In Final Cut Pro X, drag the newly imported SmartSound Music Clip onto the existing SmartSound Music Placeholder and choose either “Replace” or “Replace and Add To Audition”. Round tripping!
When FCP X came out in 2011, it broke the old Sonicfire Pro plugin. I checked with the company a few times and was told they were working on it. I think the interest has been there all along, it was just a matter of figuring out how to implement round tripping with FCP X.