Situation: Crossbones is on his first click, currently has an action token and pushes to attack an opponent. He successfully hits and damages his opponent. He then applies pushing damage and I Shot Cap appears on his dial.

Q: In this situation can Crossbones now, immediately use I Shot Cap?

I'm leading towards yes, but I want to be sure...

No. He did not possess the power, so it could not activate, when the triggering event occurred. Crossbones needs to possess this special power...

I SHOT CAPTAIN AMERICA - YOU'RE NEXT: Once per turn, when Crossbones hits with a ranged combat attack, he can use Sidestep and modifies his attack and damage values by +1 this turn. After he uses Sidestep and actions resolve, he may make another ranged combat attack against a different opposing character.

In your example, he didn't possess the power until after the action resolved and not when he hit with the attack.

Thanks NV