These are the 6 constables who were first mentioned in Adventure 2. Three of them might have crossed swords with the party in Adventure 3.
In Adventure 5 the players get a chance to play a few scenes from the B-Teams perspective, to see things that the main party is probably too busy to deal with. They also give you, as GM, an opportunity to horrible murder some good guys without derailing the campaign the way a normal TPK would.
I suppose you could create new characters to fill the same role as the B-Team, but our intention is that they are NOT player characters. The players don't get full agency over what they do. It's just a way to show 'cutaway scenes' without the players sitting around bored while you describe things.
The GM says, "The B-Team is escorting dignitary X, and they're in the Cloudwood. You see a monster doing something nasty. What do you do?" The players don't get to say, "I wouldn't go there." They just get control over these NPCs during a few scenes.
So I'd be wary of letting them make their own characters, because they'd probably expect to have more control than they really need.