Presentations provides several options for copying slides from other presentations to the current presentation. One option is to simply open both presentations and use the copy-and-paste method. Alternatively, there are two commands that can be used to copy all slides of a presentation in one step:
In the mini sorter, select the slide behind which the slides are to be inserted.
Then click on the icon's arrow of the command Insert | Slide 
and select Slides from presentation or Slides from outline from the dropdown menu.
The difference between these two commands is that Slides from presentation inserts slides of a normal presentation file (Presentations or PowerPoint file) and Slides from outline inserts slides of presentations that have been saved as an outline in RTF format.
A file dialog appears when you choose one of these commands. Select the desired file and confirm that it has been opened. The entire content of the selected file will then be inserted into the current presentation.