In that case you could theoretically choose to fight as if it were the fight phase, but it wouldn't do anything because you aren't within 1" of an enemy. When you fight as if it were the fight phase, you still must go though all the steps, and the first step is choosing a unit. Since the unit isn't eligible to fight, you cannot choose it. You'd be better off choosing to shoot instead. The reason the whole "pile in when fighting a second time even if I'm not within 1 inch" thing works is because the unit charged that turn and thus is eligible to fight even if they aren't within 1" of an enemy. Since the Knight died to overwatch, it never made a charge move thus cannot have successfully charged.