[HELP] Install Civ 5 from a retail copy without a disc drive

[Professor Noir]

Hey guys,
I got Vanilla Civ 5 from steam, loved the game, and I want the expansions. The complete edition for me now is $39.98. On Amazon, I saw the retail copy of the complete edition for $15.00. One problem, I didn't put a disc drive in my new starter gaming pc. Can I just download the game through steam using the Cd-Key on the box, or will I need to use the disc? If I can use the key, will it not download properly since I already have the base game? Thanks in advance!

[The Rock God]

All you need to do is activate the key.