For purposes of giving hints to the plot to come, you probably should add in a few side quests of your own design along the way, to let your players know that something big is on the horizon. Just how much is a tricky balancing act, yes, but 1 level's worth of exp would seem fine. If you have +1 APL, it's not that hard to balance, but if you add much more than that, then it can be hard to get back on track.
A full 1-20 campaign can take years to complete though, so I do wonder whether or not you'd be better served trimming out the RotRL content you don't need for your own story. You could, for instance, have your party start with the third or fourth episode, at the appropriate level for the one you chose. This would allow your characters to have more detailed backstories and specific ties to your own campaign threads, with reasons for them to be distracted by the events of RotRL on their way to even greater threats.