Not yet. While I realize that iTunes isn't a JRiver product they have provided us with "Export to iTunes & iPhone" and "Export All to iTunes & iPhone". Unfortunately neither of those does what one might assume they would do, take data about the songs in our MC libraries and import that data into the iTunes database (ratings, play count, skip count, last play date, etc.).
I have/had been using this excellent iPhone app called "SmartPlaylist" which allows me to select music from my iPhone library much like the Smartlists in Media Center. But SmartPlaylist is worthless when the data isn't available, and "Export to iTunes & iPhone" and "Export All to iTunes & iPhone" just doesn't move those MP3 tags and other data.
I have managed to restore a program that was available many years ago call MCiTunesSynchronizer to limited functionality. That executable when run from a batch file can extract data (ratings, play count, etc.) from Media Center and apply it to a matching record in your iTunes library. Updating iTunes then becomes a multi-step process. Add new songs to your Media Center library, go to iTunes and re-add the folder with those new songs into iTunes. Once the music is in iTunes run MCiTunesSynchronizer to transfer the MP3 tags to iTunes, which then can be synchronized to one's iPhone.
JRiver should tackle this problem.