If you purchased the eAccess (non-Steam), you have paid $10. You may upgrade to the $30 core module for $20.
Simply click here, agree to the disclaimer, then instead of creating a new account on the second page click SIGN IN.
After you sign in with your Rogue System credentials and followed the on-screen instructions, you should only be asked to pay $20.
Once you own the Core Module, you can then use the Steam key migration form to get your Steam key for Rogue System on Steam.