Yes. Just because it presumes you're creating a new one means nothing. You can still use the principles for creating a new one to alter an existing one.
I don't use Stationery, since I don't normally use 10.5 or 10.6, so I couldn't tell you for certain. However, the second URL I gave you links to a detailed explanation of how to modify an existing template, so if you haven't read that, you should, since I believe it will solve your problem.
This thread has been closed by the system or the community team.
You may vote for any posts you find helpful, or search the Community for additional answers.
edit existing Mail stationary
Welcome to Apple Support Community
A forum where Apple customers help each other with their products. Get started with your Apple Account.