Simple answer is no.
You have to do it with your mouse
Since you can assign a key in the setup for changing to 2hander, I guess you can do it for left hander too or am I wrong?
He's asking, at least i understood it that way, if there's a macro to swap the OffHand to a Shield/another OffHand from Inventory.
Else it wouldn't be a prob -.-
Ahhh, well if that is the case, then no, that can't be done.