Edit "DCS World\Mods\aircraft\FA-18C\Input\FA-18C\joystick\default.lua", and put this in on line 8 (below the line that starts with "join")
{ down = hotas_commands.THROTTLE_SPEED_BRAKE, up = hotas_commands.THROTTLE_SPEED_BRAKE, cockpit_device_id = devices.HOTAS, value_down = -1.0, value_up = 1.0, name = _('Speed Brake Switch - EXTEND/RETRACT'), category = {_('_Custom'), _('HOTAS')} },
it should add a new keybind "Speed Brake Switch - EXTEND/RETRACT" in a category called "_Custom".
This will extend the speedbrake when you push it, and retract it when released. Making it perfect for the x55 "slider"/speedbrake switch.