Sure can. Open your main presentation, click "Insert", "Slides from file",
browse to and select your second presentation. Click the "Insert All"
button. If you have PowerPoint 2002 or 2003 you will also have a check box
that says "Keep source formatting". if you check this before you click
"Insert All", the design of Presentation 2 will stay the same when merged
with Presentation 1. If you do not check this, it will take on the design
of Presentation 1 (the one you are merging into).
If you have PPT 2002 or 97, check out the following:
http://www.rdpslides.com/pptfaq/FAQ00041.htm