I just started poking around with this game, and I really dig making use of the special "Defensive [supporting] Fire" mechanic with Archers. To abundantly clarify, by "Defensive Fire", I do not mean simply fighting one-on-one against an attacker; I mean how an archer unit can be placed behind another friendly unit, and when that friendly unit is attacked, the archer unit fires on the attacker, from behind the front line.

In the manual, I have noticed the magic items "Magical Bow" and "Magical Crossbow", which state "Gives a unit a Missile attack rating of 5 or increases a unit's Missile attack rating by +2", and "Gives... 8 or... +4", respectively.

So, if I gave that Magical Bow or Magical Crossbow to my unit of, let's say, Apprentices, who are positioned behind a Warband, then if the Warband gets attacked (by melee), do the Apprentices do the Supporting/Defensive Fire thing the same way Archers would?

Thanks in advance!

No, it is only Archer units have such an ability.
The artifacts just give an additional weapon profile, not the ability.