Sure. Just grab the existing slide and do a File/Save As. Then in the Save
As dialog box, select an image type in the "as type" dropdown box at the
bottom. PNG usually works well.
Then go to the presentation you want to add it to and use
Insert/Picture/From File. You can do this on the slide master or on an
individual slide. Draw/Order/Send to Back so it's underneath the text
placeholders.
Or you can use Format/Background/Fill Effects, go to the Picture tab, and
choose the PNG (or whatever image type) you just created of the existing
slide.
Does that cover it for you, Jan?