what type of interactive form are you trying to combine? The following document outlines the constraints and rules around combining Form based PDFs. See chapter 4 in this documentation here: http://help.adobe.com/en_US/livecycle/11.0/ddxRef.pdf for LiveCycle Assembler.KJ

Dynamic Interactive Forms cannot be combined with other PDFs. You will need either LiveCycle Assembler or LiveCycle Output to flatten the interactive form, and then it can be merged with other non interactive pdfs. If you don't have LiveCycle, one thing you can do in Acrobat is print the form to the...