I'd need a lot more info to respond to this, and a pic wouldn't hurt.
Is this an abandoned box, or are any wires in the box to enter the disconnect? I can't imagine 100-amps worth of conductors fitting a single-gang box.
If I had to venture a guess now, I'd say to mount an extension box over the existing box, mount the disconnect to one side, and nipple between them.
Fine Electric Co.