I just spotted this on Etienne's blog - Joel Semeniuk has announced that an alpha version of his VSTS Process Template Editor is now available on a GotDotNet workspace here.

There are blog articles about customizing process templates by hand - and for simple tweaks, editing some of the XML is very straightforward. However, if you're really trying to do something more complicated, or making substantial changes to the process guidance in the shipping methodologies, then a tool like Joel's is going to be very useful.