There's a thing called a travel agent that runs the brake cable around a wheel so that the pull on the way out is different to the pull on the way in. My Trek 520 years ago had these to make the V-brakes work with road brifters. I should imagine you can just reverse that things to make them work the other way (ie, make your V-brake levers work with caliper brakes). Maybe this is what your mechanic is thinking of