Sure. If you know what you're doing and are handy with a soldering iron and a schematic, it could be done. Otherwise, leave the job to a competent tech.
Throw a spring reverb into the amp circuitry. Where and how you put it in is up to the amp, the schematics and the person doing it. If you don't know what you're doing, find a tech that can do it.

Or get a really big echoy room.