Can I change the error messages sent by Postfix?

Unfortunately this is a feature only introduced as of Postfix 2.3 where there is a new template feature available.

Mac OS X 10.5 Leopard Server includes Postfix 2.4.3 which can handle templates. (see "man 5 bounce")

Mac OS X Server 10.3.x/10.4.x comes with Postfix 2.1.5, so you are left with 3 solutions:
1. Modify the source code of the version provided by Apple and recompile it.
2. Install Postfix 2.3
3. Upgrade to Mac OS X 10.5 Server

1. and 2. are tricky and unless you really (I mean really) know what you are doing, this is not recommended.
The Postfix version that comes with OS X Server has been modified to play nicely with Apple's authentication mechanism, so a plain vanilla build will not work.