Can I add slides to an existing ActivePrez file?

Yes you can. There are multiple ways of achieving this depending on your needs. For one off ad hoc slide insertion, a separate PowerPoint file can be inserted as an object icon and when you click the icon on the icon, the slides you embedded are shown. For more permanent additions, you can insert slides in to the structure of your ActivePrez and an integrated help system is provided in the software to show you how to do this.