Your dealer can also get you the keypad radios for about half the cost than your irrigation dealer.
I just checked your license to see if they set you up with spare frequencies when they set-up your irrigation (they didn't). If they had, you could set all your radios up with a "talk" channel separate from your irrigation. Nothing worse than keying in a code only to have someone interrupt with conversation. Unfortunately though, you would have to apply for and then receive another frequency for this to work. If you decide to go this route, get a few extra frequencies so that the pro shop could have their own as well. Just a thought.
You can check your frequencies here:
http://wireless2.fcc.gov/UlsApp/UlsSearch/searchLicense.jsp, or take the radio to a radio shop and they can do it as well. This may be more feasible since some radios are secured with a PL code that you would have to know for the additional radios to work with the existing.