I am not aware of any. Sony Madison has never tried very hard to garner third-party support for their products (although they have done a little better this past year). I am not aware of any support or other third-party stuff for DVDA, other than the VASST training materials.
You can search the Internet for DVD menu PSD files for Encore and DVD Lab, two programs that have LOTS of support. I've also found PSD (and similar format) files at Ulead for their line of programs. You'll have to learn to import layered PSD files, but the results can be very professional.