Hawley, 30, was a free agent having been released by Notts County in the summer. He has penned a deal to stay at Glanford Park until January 7.
Clarke will join on a three-month loan from Charlton Athletic on Saturday morning.
Both will go straight into the squad to face the Blades on Saturday, with Clarke taking the No.9 shirt recently vacated by Bobby Grant and Hawley wearing No.22.
Source: FOOTYMAD
