As I just answered this question for a customer: of course it is possible to digitally sign Office Open XML file format packages. This is a core part of the Open Packaging Convention which is based on open standards such as W3C XML Digital Signatures and X.509 certificates. You can even apply more signatures on one document - programmatically or directly from within Office (Word, Excel or PowerPoint). For more information take a look at the following links:

These are just a couple of interesting links. Of course if you are interested in all the details of the standard, the Open Packaging Convention part of the standard is the place to look at:

Hope that helps others as well;)