If you are an early user of Sky Nations chances are you already have an account you registered and have also received a Steam key. If you would like to use this account on Steam and not create a new one. Downloading and install Sky Nations on Steam and simply click the Link Account button when you first Sky Nations rather than entering a new username.
You will then be taken to a new menu where all you need to do is enter your Sky Nations account details to link it to your Steam account. After this process you will automatically be logged into this account every time you start Sky Nations via Steam.