That's the question. You CAN Go Aggro; I'd allow it. In movies or books, there'd be a chase to the shed and then a beat where the dude figures out hes trapped and has no gun and there's a dude with a shotgun between him and the door.
Now it's up to the NPC. Is he really gonna try to fight his way out, unarmed, against a dude with a shotgun?
No? Is he just thinking about it? Then yeah. How is the PC trying to take him hostage? Probably by talking to him or threatening him with the shotgun to stand down and give up. Go Aggro.
But let's say that he's gonna fight, unarmed, against a dude with a shotgun. Then yeah, we're Seizing him By Force, as a hostage. How are you doing it?
"Well, I don't want to kill him. If he tries to get around me, I'm gonna butt him with my shotgun, right in the head." Roll+Hard and he gets only 1 harm by default, 2 with terrible.
Or "Hell, if this dude tries to get around me, I'm gonna get blastastic on him. Hostages work better if they're already bleeding!". Roll+hard and the dude gets blasted.
There's always option away from the mechanics, in the fiction, that influence the mechanics. Don't want to shoot him? Then you're not Seizing with the shotgun.
It's all Up To The Fiction.