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.
The Iron will still be without defender Paul Reid (hand), Callum Kennedy (toe) and Damien Mozika (ankle) for the game.
Sheffield United will welcome back Darryl Westlake (thigh), meaning he could make his first appearance of the season.
Deadline-day signing Dave Kitson will not be considered for selection as he is still working on his fitness.
Fellow striker Chris Porter remains sidelined with a knee problem and is not yet ready for a return.
Manager Danny Wilson has no other injury worries and is likely to name an unchanged line-up from the side that beat Bournemouth 5-3 last weekend.
That means strikers John Cofie and Nick Blackman should continue a partnership which has yielded four goals in their last three games.