alright, i think .xsi is some kind of XML standard, I'm not quite sure. You're probably better off checking
www.oooforums.org, unless someone else here has done this before. I know that the PowerPoint format isn't readily available, so you might need to create a php-script which gets called by a vbscript from within PowerPoint. That's the first thing that comes to mind. There might be an easier way. Post your progress please, I'm interested in how this is going to work out.
Good Luck
Dizz
ps. my last comment was not supposed to sound conceited, hope you didn't take it that way.