You can do this with a choice, it will look something like this:
![0EM50000000QnYZ.jpg 0EM50000000QnYZ.jpg](/t5/image/serverpage/image-id/1113i98FBBB8A37879E54/image-size/large?v=v2&px=999)
If the opportunity stage isn't available as a field for you, I'd use the RCM to set a revenue stage, or you can might be able to use "Has Opportunity' or "SFDC Type" to make the proper distinction.