Thanks for the follow-up, yawor!
The question is more one of whether the MX-500 and Flirc will get along. As I understand it: The Flirc can be taught to send just about anything to the device in response to just about anything received via IR? But some newer IR devices have inter-symbol timing that wasn't foreseen by URC's engineers, or operate at frequencies that were not in use at the time the MX-500 was made. (The MX-500 applicable frequency range of learnable infrared signals is 10 to 100kHz. The Flirc operates at 38KHz, I believe?)
No, you cannot directly enter hex codes into the MX-500. Only thing you can do is use a JP1 (or, of course, other remote) to teach it. I think I read where somebody tried that with a Harmony remote and got the same results as I: The MX-500 indicates it's learning, but does nothing with the results.