I am a bit confused by your original statement.
You said the letters are currently programmed by moving an arrow around the keyboard on screen but then you talked about a number pad with 3 letters per digit on the remote.

Is the numberpad an alternative method on the original remote or is it just the method you are looking to change to?

If it is then latter then anthony is spot on the only way you could do it is to have an individual key for each letter and program it so that when you hit it,it jumps to a known start point on the screen then adds he correct number of up, down, left, right moves to navigate to that letter then it sends enter.