I've GM'd and played MERP and Rolemaster rather a lot, here's my advice.
Although quite similar in appearance there's no guide (That I know of) that allows easy migration between Rolemaster and MERP; one of the principal problems is that there are several versions of RM; this counts double for the RMSS as that's sufficiently different that migration is a pain.
I personally migrated to using all the Rolemaster rules from the get-go for MERP after the first level up pain and I'd recommend that for the future (Or now if your characters are low level) however you have to study what's available in Rolemaster so it's themeatically correct and that can be a LOT.
You have two options that I can suggest:
Extend the existing levels. (Recommended) Go through and add another 5 levels for MERP using the Rolemaster source material; Meaning add some new spells (picked from Rolemaster) and new stuff from the fighters (who frankly just get larger OB's anyway, there's no new feats or anything to worry about) - This is a bit of work for you, but much less for the players - it also means you can keep playing with the rules you're already familiar with as Rolemaster does mean some changes.
Migrate wholesale to Rolemaster. This is a lot more work for you and the players; in the future if you run more games this is the way to start, but upgrading is a pain. Each character will effectively have to be done from scratch as there are skills, packages, spells and options available (which will be familiar) But it's the only way to really do it; all the skills that have previously been picked can be used as a guideline but you'll have to check over what's available and it's a lot of work.
Overall I'd highly recommend just house-rule to extend the levels available to MERP for the characters in use, it's a lot simpler in the long run and if you want to do Rolemaster then save it for the next campaign.
If you have trouble with ideas/concepts/spells with MERP/Rolemaster feel free to ask another question or send me a message, I've been running both games for a long time.