There are 2 ways I was thinking about, and PROBABLY the 2nd one would work better. So here we go.
for i = 1,1000000 do
if PedGetModelID(i) == modelID then
PedSetWeapon(i, weapID, ammo)
end
end
The second method is editing the pedstats.dat. I do not know how to edit their values via LUA, other than only overriding 1 ped's stat.