You can use:

ActivePresentation.Slides.insertfromfile "path to file", x, y, Z
x= where to insert
y =first slide to insert
z = last slide to insert

Note you will have to do more if you wish to retain formatting. Have a look
at Shyam's site here http://skp.mvps.org/pptxp001.htm

Also be aware that in 2007 vba  there's a bug which give red x's instead of
graphics with insertfromfile if you insert from a .pptx / .ppsx file