I'm having trouble including pages from an external pdf file into my tex-file. The thing is, I use LaTeX to automatically include names/addresses into letters (which are sent to lots people at a time, so I really don't want to be doing this manually

Basically, what I'm wanting to do is this: import a pdf page into LaTeX and be able to use it as a sort of "background" over which I can just print my own text (that is, those addresses). I know it accidentally happened to me a long time ago, but I can't seem to be able to reproduce that effect.
So far, I've used the pdfpages package and the includegraphics package. All I'm getting now is:
page 1: the page that is imported from the pdf
page 2: my own text
And I'd like to see them on the same page, layered!
Is there any way to do this? I'm working in the letter-class.
Thanks in advance,
Yfke