You will need a description.ext file for that.
Here is the part of the description.ext file in my mission that deals with this:
class Magazines
{
   class AK74
   {
      count = 12;
   };
   class GrenadeLauncher
   {
      count = 3;
   };
   class PK
   {
      count = 5;
   };
   class RPGLauncher
   {
      count = 3;
   };
   class AT4Launcher
   {
      count = 1;
   };
   class Mine
   {
      count = 2;
   };
   class Pipebomb
   {
      count = 2;
   };
   class Tokarevmag
   {
      count = 24;
   };
   class 6G30Magazine
   {
      count = 2;
   };
};
class Weapons
{
   class AK74
   {
      count = 3;
   };
   class AK74SU
   {
      count = 1;
   };
   class AK74GrenadeLauncher
   {
      count = 1;
   };
   class PK
   {
      count = 1;
   };
   class RPGLauncher
   {
      count = 1;
   };
   class AT4Launcher
   {
      count = 1;
   };
   class Tokarev
   {
      count = 6;
   };
   class 6G30
   {
      count = 1;
   };
};
the 'class WeaponName/MagazineName' is the name of the weapon or magazine you want to add. The 'count = n' is how many you want.
-Pilot
EDIT:
Here is a good description.ext tutorial for you.