We have a mixed-mode Exchange Server organization. We'd like to create a recipient policy to stamp a secondary SMTP address on all recipients, whether they're homed on Exchange 2000 Server or Exchange Server 5.5. Is there a way to do this?

No supported way exists because Exchange 5.5 doesn't really have an equivalent of Exchange 2000 recipient policies. Exchange 5.5 has a Site Addressing object, which is similar but lacks the ability to stamp secondary proxy addresses on recipients. You might be able to use ADSI Edit to edit the Exchange 2000 recipient policy that applies to Exchange 5.5 recipients and configure appropriate connection agreements to replicate to the Exchange 5.5 side the changes that Exchange 2000 applies. Remember that Microsoft doesn't support this approach, so if your Exchange 5.5 recipient policy goes awry, don't go crying to Microsoft.