yeah. Your Ammo pack must have an Impact event so it starts if the Player touches it and the code would be like.
In this code the skill1 from the player is his Ammo

Code:
function ammo_event()
{
  if(event_type == EVENT_IMPACT)
   {
     if (you == Player)
     {
      if(you.skill1 < 150)
      {
        you.skill1 += 50;
        you.skill1 = min(you.skill1,150);//The maximum is 150
      }
     } 
   }
}