You could use give_item with the ammo_ items, although it is by far not recommended. If, however, the weapon the players have is a fixed one (a M4A1, for instance), then you would know in advance the amount of ammo a ammo pack has. If that is the case, you can do a call as simple as this one:
PHP Code:
new ammo = cs_get_user_bpammo(id, CSW_M4A1) // His ammo right now
ammo = min(90, (ammo + (get_cvar_num("how_many_packs") * 30))) // Truncate his ammo to the maximum allowed
cs_set_user_bpammo(id, CSW_M4A1, ammo) // Set his new ammo
Note the use of function min to truncate the amount of money the player has (he can't have more than 90 because it's M4A1's maximum bpammo).
The killing part should be taken care of by Ham_Killed.
Moreover, it is recommended to cache the CVAR in a global variable at new round or something alike (you can look at the forums for this).