How to get letters from hardware keypad?

Hi I'm coding an app for a android device with an hardware keypad.

The keypad is somwthing like this one : http://www.tradeindia.com/fp2133898/Jepower-HT380A-Quad-Core-Android-4-2-2-Rugged-Industrial-PDA.html

iI have disabled the android keyboard,and when I press a key on a TextField it inserts only numeric the numeric value of the key, I can't find a way to populate the TextField with the letters (for example pressing the 2 key I need to populate il with a,b or c letter)

Please do you know how to achieve to a working solution? thanks