The problem with adding encounters to Zone 21 is you don't want the npc ships flying around in the minefield, they'll just explode. Trick is to create a zone in the minefield around the alaska jumpgate. I have the gate open in our mod so i did this,
[Zone
nickname = Zone_Alaska_jumpgate
pos = 79950, 0, 12656
rotate = 0, 0, 0
shape = SPHERE
size = 8888
comment = Around Alaska jumpgate
sort = 51
toughness = 19
density = 3
repop_time = 15
max_battle_size = 4
pop_type = jumpgate
relief_time = 15
population_additive = false
faction_weight = fc_n_grp, 1
encounter = area_nomads, 19, 0.200000
faction = fc_n_grp, 0.800000
This works a treat as the nomads only spawn in the area without mines inside Zone 21, hope this helps