For players or AI?

If its for players you could just add the magazines to the

initPlayerLocal
and
onPlayerRespwn

The first is when a player joins the game, the second when they respawn. Have to create both of these in your mission folder, then add the addMagazine command with the correct count and type.

For AI depends how you spawn them, you could use the spawn script and add a sleep of a second or two then add extra mags, so every time its called to spawn a group that group will have extra mags added.

Theres other options too but to actually have them appear with the extra mags when placing them in the editor... i think you will have to edit there config files to change the default amount.