http://openxmldeveloper.org/articles/110420.aspx
Merging PowerPoint Presentations
This article will show you how to programmatically merge PowerPoint 2010 presentations into one using Open XML, Open XML SDK 2 and Visual Studio 2010.
We have 3 PowerPoint 2010 presentations containing a calendar for each year from year 2011 to 2013 respectively. We’re going to merge them in to one presentation using a template.