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.