You know what, K9 is based on the stock app...I think. So basically, K9 is the answer to your question. Forget about the stock app. You can find K9's google code page where you can download newer versions, report bugs, etc.
I have not tried K9 yet, but today I found "Inserty" in the Market (free) and it works great for inserting signatures in email on my Droid1. It inserts your custom text items into any app.