Will the users have Acrobat or just Reader?
Acrobat pro XI
Ok, spent a better part of the day trying to figure this out, but found the answer twenty minutes after finally asking the question:
That's the same as clicking the Insert Pages button in Acrobat.
If you really want to control these documents you should either use a script to insert them as specific locations (using the insertPages method), or have the users attach them to the PDF, instead of merge them with it.