Is it still possible to join in?
I suppose. You will change roles with a random person.
Huh? Wont that mess with the balance, or something like that?
If you switch with an investigator or doctor, then we will have one townie who knows that runman has a special role, which undermines some mafia tactics.
If you switch with a mafia or the SK, we will have one townie who knows that runman is anti-town, which will get him lynched instantly.
The only thing you can do that won't unbalance the game is to make runman a townie, but then everyone knows his role anyways.
EditI guess you could switch around three or four roles though.