I wanted the same thing. When I discovered I could send text messages via APRS to a callsign on my kenwood tm-d710 I felt excitement that was then followed by disappointment in that I had to type my message using a knob selector. I had initially thought you could use the number pad on the microphone to input the message T9 style.
I think someone made a keyboard for the older model but it didn't work with mine
I think someone made a keyboard for the older model but it didn't work with mine