They don't allow you to directly access the game with an account that wasn't created through their steam client. You can bypass it by going into the steam install folder for the game and opening UPDATERUSA.exe. From there you can login with your old account. You just won't be able to make purchases through steam and have to make them through warp portal.
Any halfway decent programmer would have made a way for existing account to connect.
Also if you do decide to make a new account through the steam client, then open the game through the default steam launcher. Prepare to receive a 1001 error. They released a Warp Portal launcher some years ago. The basis of the steam client is this launcher. In all that time they have not figured out how to stop that error from happening. It happens when the hardware gets overloaded and crashes. It then doesn't reveal itself for authentication on reboot. Usually the staff does not notice it went down for days to months. Of course you can also bypass this by once again opening UPDATERUSA.exe in the client and using the account information used when making the account through steam.