Bernie, on my Pa900, the procedure is as Steve A has written - but if I follow correctly (and I'm not on my kb at this time) that would give you however many bars you created of silence ... if you perhaps want to keep the bass and/or drum pattern of the SMF going, you would have to COPY the measures you want, INSERT them where you want, and delete any unwanted tracks from those measures ...
It all depends on the end result you are looking for ...