It is not possible to add an AT missile with addmagazine command to a soldier that has an AT launcher but no backpack.
This must be due to the fact that addMagazine command is trying to add to player inventory (uniform/vest/backpack) and since the soldier doesn't have a backpack and since the AT missile doesn't fit inside vest/uniform the AT missile is not added at all. The expected behavior would be to add the missile to the AT launcher.