Winger Worrall, 23, joins from fellow League One outfit Rotherham, while 29-year-old midfielder Harkins, 29, switches from Scottish Premiership side St Mirren.
Worrall, who only joined the Millers in the summer, has been dogged by injuries and has had no game time since August.
Harkins has also been allowed to leave his club after just a few months following boss Danny Lennon's accusations that he did not work hard enough.
Both men are in contention for Sunday's FA Cup clash with Liverpool and they follow Adam Lockwood through the arrival door at Boundary Park after the centre-half penned a short-term deal earlier on Friday.
- Sports & Recreation