Doing this with your editor would be an easy way, like Dave said. Otherwise I would add an intermediate step and render out the first part and the looping part separately, then import them into a new comp, add them both to the timeline and set the looping clip to loop a whole bunch of times in the interpret footage dialogue (right click on the video -> Interpret Footage -> Main)
…which is pretty much the same thing you would do in an editor, just in AE..