I tried to add custom magazines to unit with this:
this addmagazine ["30Rnd_65x39_caseless_green",10];
Normally this would result in giving the unit 10 clips of a certain type. The 10 clips however counts as single bullets, so 10 bullets total in this example. In this case adding more then '30' makes no difference, it wont go past one magazine. Adding this code multiple times in the unit init does add multiple magazines to the unit.
It is possible that I am wrong, but I am scripting a bit now in a mission and remember this to work in arma 2.