Find default.lua at C:\Program Files\Eagle Dynamics\DCS World OpenBeta\Mods\aircraft\JF-17\Input\JF-17\joystick and add the following two lines under the T1: MMS
{down = key_cmds.HOTAS_Throttle_T1_Forward, up = key_cmds.HOTAS_Throttle_T1_Center, value_down = 1, value_up = 0.5, name = _(cmd_names.HOTAS_Throttle_T1_Forward), category = _('HOTAS_TOGGLE')},
{down = key_cmds.HOTAS_Throttle_T1_Backward, up = key_cmds.HOTAS_Throttle_T1_Center, value_down = 0, value_up = 0.5, name = _(cmd_names.HOTAS_Throttle_T1_Backward), category = _('HOTAS_TOGGLE')},
You create a new category HOTAS_TOGGLE, there you will assign BOAT switch front and back position to A/A and A/G respectively. The middle position will be the NAV mode.