If you want to add an existing presentation to a presentation you're working on right now, just insert the slides from the existing presentation. You can insert the entire presentation or just a group of slides.
Click on the Slide Sorter View button. The display will change to Slide Sorter view.
Click in the space where you want the slides from the other presentation to appear. The insertion bar will appear in the selected location.
Click on Insert. The Insert menu will appear.
Click on Slides from Files. The Slide Finder dialog box will open.
Type the path and file name of the presentation that contains the slides that you want to add to the open presentation.
Click on Display if the slides do not appear automatically. A preview of the presentation slides will appear in the Select slides area.
Click on the slides that you want to insert into the presentation. The slides will be selected.
Click on Insert. The selected slides will be inserted into the open presentation.
Click on Close. The Slide Finder dialog box will close.