Who is the manufacturer of your keyless system now, it may be expandable to add a key pad.
I have an Essex KE1601 system with the keypad, it is obsolete so if I have a problem I will have to upgrade to the Essex KE1701. The KE-1700 Series Keyless Entry System
Trimark also makes a keyless system.
We've become so dependent on the system I would not like to think about not having it.
Installation seems fairly easy if you have an existing system, hardest part will be getting access to the control module and then routing the appropriate wiring harness. Depending on how your current grab handle is mounted it might be possible to replace with a keypad type, if not you can go with an separate external keypad.