If I understand the scenario correctly, the SonOff mini can do this (or any other smart switch which can run Tasmota firmware).
The SonOff mini exposes a GPIO pin and a local reference to be used as a 'toggle-on-change' input. This has the advantage of the firmware always knowing the relay state, being possible to define a default power-on state (depending on the firmware), and providing full 3-way switching.
You retain the existing 2-way circuit, but it must be isolated from all other circuits. The SonOff mini requires a live and neutral feed, and generates a switched live for the lamp. The existing 2-way circuit will operate as so either switch toggles the 's' input the SonOff - this is even documented in the installation instructions as an option.
Remember, one 'S' terminal is half-mains voltage, the GPIO input is 3v3 above this - so treat the wiring just as if it was a conventional lighting circuit. You need somewhere close to the fixture to fit the SonOff device.