This will probably be in next update. Meanwhile just add this line to input/joystick/default.lua
{down = 3006, up = 3005, cockpit_device_id = devices.NAVIGATIONPANEL, value_down = 1.0, value_up = 1.0, name = "T1 Fix (Two-stage trigger middle position)", category = "Radar"},
and map it to the half-press stage on your trigger, and then the TV-fix for three position switches on the full-press.