Go somewhere that has air strikes munition. Approach the air strikes munition and press Y to steal it.

Then run for a few block but do not run too far, keep the air strikes munition picture still displayed in your map.

Then call for a resource pickup. Wait until the helicopter has appeared in your map, then quickly run until the picture of the munition has disappeared from your map.

If you've done the trick correctly, the munition will disappear even though the hellicopter is not yet taking the munition and it will be added to your stockpile.

If you go back the munition place, the munition will be replaced with another, or the same type of munition. So you can do the trick whenever you want to.