Anyone doing work to the level you are should consider investing in a good quality mig welder with good features.
It is, by far, the preferred way to repair holes in a firewall.
That said, and to answer your question directly, there are professional products to make the repair without welding. It's not going to be cheap though.
http://3mcollision.com/3m-panel-bond...ive-08116.html
Eastwood makes some stuff too but I don't think you can state that it's a place pros buy from:
http://www.hotrod.com/articles/sheet...ue-you-can-do/
If you really wanted to go cheap (and you will probably take some flak for it), you could use JB Weld. I wouldn't give advice to someone to use it for that but my thought is that it would probably work OK if you prepped the metal by scuffing and cleaning well.