Not sure if you are asking, or have found out already. You use one of the move commands to have a unit switch places in the setup phase (only inside the setup zone, or it will just plot a move command).
Ammo bearers are typicly keept close to the respective weapon they are serving, to shear with it the extra ammo they are carrying. They can also be used to go and aquire extra ammo and still have the main wepon stay in place.