In Medieval 2 it ain't possible to do that.....
But you can edit the file:
type Desert Archers
dictionary Desert_Archers ; Desert Archers
category infantry
class missile
voice_type Light
banner faction main_missile
banner holy crusade
soldier Desert_Archers, 48, 0, 0.8
attributes sea_faring, hide_forest, very_hardy, can_withdraw
formation 1.2, 1.2, 2.4, 2.4, 3, square
stat_health 1, 0
stat_pri 7, 1, composite_arrow, 160, 25, missile, missile_mechanical, piercing, none, 25, 1
;stat_pri_ex 0, 0, 0
stat_pri_attr no
stat_sec 3, 1, no, 0, 0, melee, melee_blade, piercing, sword, 25, 1
;stat_sec_ex 0, 0, 0
stat_sec_attr no
stat_pri_armour 0, 1, 5, flesh
;stat_armour_ex 0, 2, 0, 0, 1, 5, 3, flesh
stat_sec_armour 0, 0, flesh
stat_heat 0
stat_ground 1, 2, 0, -4
stat_mental 3, normal, untrained
stat_charge_dist 30
stat_fire_delay 0
stat_food 60, 300
stat_cost 1, 370, 100, 45, 105, 370, 4, 110
armour_ug_levels 0, 1
armour_ug_models Desert_Archers, Desert_Archers_ug1
ownership moors, egypt, slave
era 0 egypt
era 1 egypt
;unit_info 3, 7, 6
recruit_priority_offset 0
bold part # of ammo
stat_pri 7, 1, composite_arrow, 160, 25, missile, missile_mechanical,