PowerPoint 2007 does not have any of the end-user user interface for adding custom XML schema like Word 2007 does, although it it something that we would like to eventually add in an upcoming release.
At the moment, your best bet is to utilize the object model or the file format itself. The class you will be interested in is called CustomXMLParts and can be found off of the Presentation object.
Please let me know if you need more information.
Unfortunately this isn't supported, although we are interested in eventually doing this. You can attach metadata to shapes in PowerPoint, but none of it will be exposed in the PPT UI.
Regarding using the PPT 2007 object model on the server, the short answer is no. The PowerPoint application is not designed to be automated on a server.
Sorry I don't have better news for you.
Regarding the question.
"I have another question, it's safe to use PowerPoint 2007 object model (assembly Microsoft.Office.Interop.PowerPoint) server side ?"
We have used the powerPoint object model on the server side and it looks find. I have the 2007 installed on the server. We programitically modify the PPT from database.
You should be aware though that the Office applications are not supported for running on a server. Here is an article that discusses the issue: http://support.microsoft.com/default.aspx?scid=kb;EN-US;257757