I found this
http://www.quiltingboard.com/t-150352-1.htm Haven't tried it yet, but it seems pretty complete - including how to get the BB-style format for the sig block.
Hmm, the page has changed since then, I think. Try this:
Go to
http://www.mylivesignature.com. Click on the wizard, and create your signature. When done, scroll down a bit to find the link that says "Want to use this signature?" The next screen asks if you want HTML or BB code - pick BB, follow the steps, and eventually you'll get your sig block code.
Lookee - it worked!! :) And it's retroactive - all my sigs got updated magically, not just new posts.