Is it feasible to add an aftermarket hydrofoil to a light aircraft?
It's been done.  David Thurston designed a system for the Lake LA-4 that was purported to allow it to land on 5ft waves.  It may have been a bit of a handful (the Bucaneer is a demanding airplane on its normal hull) and was crashed by someone who took it for a spin without Thurston's authorization, and that was the end of that.