Steam question

I have civ4 on steam, and it doesn't look lilke they plan to release bts on there. is it possible for me to buy bts retail and upgrade the civ4 on steam with it, or will i need a non 3rd party run copy of civ4 for any of that to work. thanks ^^;

They're releasing BTS on steam. You can pre-purchase it now for $27.

http://steampowered.com/v/index.php?area=game&AppId=8800&cc=US