I do not believe there is an automated way to do this because it would defeat the security of code-signing VBA Project signing.
  The two message digests are compared,
  and if any part of the file has been
  modified or corrupted, the digests
  will not match and the contents of the
  file can't be trusted. The
  verification process will fail
  regardless of how the file was
  modified - whether through corruption,
  a macro virus, or programmatic changes
  made by an add-in or Office solution.
  The verification process will also
  fail if the file wasn't signed with a
  valid certificate; that is, if the
  certificate had expired, or had been
  forged, altered, or corrupted. If
  another user modifies the VBA project,
  the Office 2000 application removes
  the current signature and prompts the
  user to re-sign the VBA project; if
  the user doesn't sign the VBA project
  or signs it with another certificate,
  the file may fail the verification
  process.
  
  Inserted from
  http://msdn.microsoft.com/en-us/library/aa190113(office.10).aspx
Code signing has the additional level of security in the fact that a developer must compile source code. A macro is not compiled and can be distributed as text. Therefore, automating macro signing would open a large security hole. Manually siging a macro is similar to Outlook prompting the user to allow programmatic access to the address book.