I have done this and I tried SendToX and it didn't work - I don't remember the error. FCPX cannot read a Premiere XML but Resolve can. I ended up exporting an AAF from Premiere to Resolve, then exporting an XML from Resolve to FCPX. See the article "How to Migrate Timelines Between Video Editing Applications":
There were numerous multicam clips and those came over; I don't remember if there were any sync clips.
It was a lot of work and required a lot of manual fix-up. I also brought over a native Resolve project -- that worked a lot better.
There are certain workflows you can make function but handoffs between Premiere and FCPX is not something you want to do except as a last resort. One constant issue I had was it didn't bring over any head/tail for any clip used in the timeline, so in FCPX each time I needed to trim or extend the clip I had to go get the original clip, bring it in and sync it manually. A lot of effects did not come over properly (not unexpected), and there were issues with some NTSC interlaced material.
In general a planned workflow would go from FCPX to Premiere, not vice-versa. FCPX is much stronger at classifying and organizing material. In a planned situation you'd not use any Fx but defer those to the final timeline edit in Premiere. Setting up the workflow would include testing what edit actions can be transferred and avoiding those which cannot be. That requires a lot of testing.
Resolve and FCPX are more compatible but even then it takes planning. A lot of people edit in FCPX and finish in Resolve but there are complications even then. For a highly planned, purely sequential workflow it is easier. But if significant edits are needed after it's in Resolve, then you must decide can it be bounced back to FCPX, or just do it in Resolve. Then if you later need to re-open the project, search for additional material, make additions, etc, the media is all back in FCPX, but the finishing edits are already applied in Resolve. In theory you could bring over a revised project to Resolve and then copy/paste the Fx from the prior Resolve timeline.
Stu Wart wrote: yes, thank you for the insights...
since 9 hrs of shooting have been synchronized in Premiere and a 4 hrs rough assembling done by assistant ... : I guess I will have to learn Premiere ! ....
Your options are use Premiere, export via AAF to Resolve and finish it in Resolve or a two-step migration from Premiere to Resolve to FCPX.
Resolve is really good, track-based but much faster and more responsive than Premiere. An FCP7 editor will generally find Resolve somewhat familiar, yet it has some FCPX-like features.
There is no guarantee it will come to Resolve perfectly but each hop to a different NLE introduces problems, so one hop to Resolve is better than two hops to FCPX.
Of course it's a Premiere project so will work perfectly in Premiere, that avoids any issues. I used only Premiere for several years and it's OK.
But if you're mainly an FCPX editor, it might be a better investment to learn Resolve because it might be more likely you'll use Resolve in the future for finishing FCPX projects. You could try the free version of Resolve, do a quick evaluation of how well the Premiere timeline comes over, then either continue with Resolve or use Premiere.