Aftermarket controllers can be installed. The controller itself needs to be installed where it can be reached by the driver and wires can be routed to it. It needs to connect electrically to 12 volt power, the brake light circuit, and the trailer harness brake control wire. It is often done as a diy item, but requires wiring knowledge and skill.
A good source of information on the work involved would be the installation instructions for a controller, which would be available on the manufacturer’s web site, and youtube videos.