Steve,
RemoteCentral, and this forum particularly, tends to be inhabited by Custom Installers.  Most of them posting here are very good at what they do, which is designing, specifying, and installing audio/visual systems.  Most of them are not, however, very knowledgeable about inexpensive sources for IR signals or infrared IR signal protocols.

A Comcast or Atlas cable remote is capable of sending any IR signal that your RF20 is able to learn, and a fair number of signals that are too complicated for the RF20.  The Comcast remotes (assuming you don't already own one) can be bought new on eBay for $6 shipped.  Depending on the particular equipment you have, these remotes can be manually programmed by entering a 5 digit EFC for each discrete signal, or if your gear is less common, you can use a $20 USB interface cable and free open source software to make the remote send any signal.  You can convert Pronto Hex or use a specification such as NEC1 device 120.47, function 63 to prgram the remote.  Lots of people teach their URC remotes using this method.

If your equipment employs very unusual IR signals for which neither the IR protocol nor the discrete signals are known, you can buy instead a learning remote such as the RCA RCRP05B (<$20), and use the interface cable to download the unusual IR signals to your computer, decode them, and then systematically try other function numbers to find the discretes yourself.

Post a question (preferable including the equipement for which you want to add discretes) at hifi-remote.com and we'll get you going.