Are you talking about the 80's/90's Ford/Lincoln systems with the rectangular keypad under the door handle ? Seems to me that retrofitting one would be possible but not altogether easy. I'd start with a T-bird or Lincoln donor & remove all the hardware. I'd also be sure to get the appropriate factory manuals for the donor as well. you'd want the body/chassis/electrical manual, the EVTM (electrical & vacuum troubleshooting manual) and the wiring schematics all of which can be found pretty easily on Ebay. Read & study the manuals to identify all the components of the systems, pull them from the donor, walk them over to your bronco & in the famous & comforting words of the Chilton corp "assemble in reverse order". I can't think of any real showstoppers to doing the swap but it IS going to take more than a little time & effort.
Off the top of my head I really don't know if Ford ever offered keyless entry as an option on any of the F-trucks or Broncos. I'm sure some of the newer Lincoln trucks have it but the doors won't be a direct swap. IF they did offer it then the swap gets a whole lot easier because you could just score a set of keyless doors as complete assemblies and retro the rest of the parts but most of the hard work will have been done for you at the factory.
Best of luck with it if and if you decide to tackle it please keep us updated, I'm sure a lot of guys will be interested to know what you learn in the process.