I concur with Larry;
About May 2015, our company converted from Primavera P6 to MS Project Server 2013. While the concept of moving a schedule from P6 to Pro sounds simple, it really isn’t. I manually exported a small (250 line) P6 schedule and imported it into project as mentioned by Georg. The result was about 10 hours of manual work to get the Project version of the schedule working correctly and producing similar results to the P6 version. Granted, much of that time was reviewing literally everything because we needed to see what was missing. I would assume that if subsequent imports were always missing the same 3 or 10 things, the import time would be reduced. The problem is that even if it is reduced, there is always going to be several hours of conversion work to make it work in Project.
Unfortunately, there are lots of minimal sounding discrepancies between the two tools that create non-trivial issues. For example, Project didn’t know what to do with the LOE tasks used in P6, half the dependency relationships between tasks disappeared, and on an on.
Treat the subcontractor as an external vendor and monitor/track their schedule based on key milestones that you have built into your MS Project schedule. They can run their schedule and their resources in P6. But you need updates to those key milestones.