DCSWorld\Mods\aircraft\Su-25T\Input\su-25T\joystick\default.lua
add these lines:
--Alternate Flaps
{down=iCommandPlaneFlapsOff,up=iCommandPlaneFlaps,name='Flaps Up - Alternate',category='Systems'},
{down=iCommandPlaneFlapsOn,up=iCommandPlaneFlaps,name='Flaps Down - Alternate',category='Systems'},
bind your controls to these new commands ingame.