You can script it but it's not necessary. You can do it in the units initialization, or in the init.sqs file. Either way lets you skip writing a new script that isn't really needed.
In your init.sqs do something like this-
removeallweapons loon1
loon1 addmagazine "HK"
loon1 addmagazine "HK"
loon1 addmagazine "HK"
loon1 addmagazine "HK"
loon1 addweapon "HK"
get the idea ?