That's exactly how I did it, just installed bell press buttons and not normal switches.
Just have a look at installing shielded cable to the sonoff if it is far away (to prevent induced voltages - dont want to take chances frying anything, maybe it will, maybe it wont)
only thing to remember is not to keep the switch pressed in for too long as this will force the sonoff into setup mode
Even we use bell switch, only way to use sonoff with original source code is by connect it to on/of switch at sonoff board. correct me if you have any other option without change the source code.