You can't do that, but you can do better
pppd (or diald I don't remember) has the ability to initiate the connexion whenever it is needed, for whatever reason (mail, web...).
So you launch firefox, you type an URL, (you wait a little bit...) and it just works!
Configuration is not as easy as configuring kppp though. I only know it can be done, and often is.
Yves.