How Do I Install A Game From Disk Instead Of It Downloading Via Steam ??

A while ago i bought Supreme Commander 2 not knowing that it was to be installed via Steam
Anyway, i installed the game & hardly really played it to be honest,
mostly due to the fact Steam would be messed up when updating.
Eventually i had to reformat my hard drive & i forgot to backup my steamapps folder.
Once again, i bought COD: Black Ops & installed Steam to my PC & added COD to my list of games.

My question is this:
How can i install Supreme Commander 2 from disk again, without Steam having to re-download everything ?
The reason i want/need to do it this way is because im using a pay as you go broadband dongle.
Downloading everything again would take ages & cost me a small fortune too.

Anyone know how to do this ?

Cheers