It can be done but it's tricky...
 Rate your music using the MediaMonkey rating functionality, which writes the rating to its database library as well as the tag.
 Export your Rockbox database using the 'Export modifications' feature.
 Open the back end MediaMonkey database using a compatible database program (I use Microsoft Access but others may work as well).
 Create SQL and/or a macro that imports the file generated by the Rockbox, joins it with the MediaMonkey database using key fields (artist, song, album, etc) and then updates the ratings field of the Rockbox database based on the MediaMonkey ratings.
 Save the modified file in Rockbox compatible format on your player.
 Import the modified database file into Rockbox.
This is time consuming and not for the faint of heart, but at this point I don't think there's another way. You have to work around a major database bug as well, see my open bug report here => http://www.rockbox.org/tracker/task/8360
I'm hoping one day a "read rating from tag" feature in Rockbox may eliminate the need for this workaround.