Stoatsa - It will NOT work - the Game (CD) Key is tied to the Steam account that it was originally activated on FOREVER - there is no way to merge Steam Accounts, or to move a Game from one account to another, this is to combat piracy and is part of the Steam Terms and Conditions that we all had to agree to. As already mentioned the only way is to purchase and add games to your existing Steam accounts.
Depending on your location it may be a cheaper option to actually purchase a boxed DVD Version of Race On and add that to your GTR Evolution account rather than purchasing GTR Evolution and adding it to your Race On Account !
My reason for this? -
Today on Steam GTR Evo expansion is £13.99, so you could upgrade your Race On Steam account for £13.99.
HOWEVER - Today on Amazon you can purchase the Boxed Race On DVD for £13.84 delivered in the UK - This means that you could upgrade your GTR Evo Steam Account to Race On and GTR Evo for £13.84 - BUT because you would be using the FULL BOXED Retail Version of RACE ON you would ALSO be getting STCC The Game, as the BOXED RETAIL DVD of Race On includes STCC (the Download from Steam does not).
Amazon UK Link -
http://www.amazon.co.uk/gp/offer-li...s=gateway&qid=1268221873&sr=8-1&condition=new