This is possible if you're willing to edit the underlying XML of the presentation or template. Here's my introductory article to using this technique: OOXML Hacking: An Introduction
An Office theme has three subthemes: Colors, Fonts and Effects. You can create a patterned fill as part of an effects theme. Then it will appear in under Shape Format in the Shape Style gallery. Here is my article about a custom effects theme that our company created, you can download and install this on your computer.
Here is a sample presentation with a diamond pattern applied to the fourth row of the Shape Style. That pattern has been applied to a shape and a chart data series on the sample slide. After unzipping the presentation, I edited the ppt\theme\theme1.xml file. The section to edit is in the a:fmtscheme part. There you will find an a:fillStyleLst part that contains three different fills. I replaced the second fill with this XML:
The first line sets the pattern to a solid diamond. Here's a reference page with the XML names for the other patterns built into Office. Then the fgClr and bgClr parts set the foreground and background colors for the pattern. Here's what the result looks like:
This just scratches the surface of editing effects themes, but it is possible.
