How do I add a png to an existing pdf in Acrobat Pro 8?

I like you guys better than tech support, so here goes; I’ve got a multi-page pdf document in Adobe Acrobat which I created out of .png files using “Combine”. Now then, how do I simply add a new png file to the document. If I try “Combine” it combines the existing pdf with the png BUT it then wants to save it as a NEW file. I just want to add to the existing file.
Thanks again, cuz I suspect this is really easy.