The 48-time capped Scotland forward is out of contract at Fir Park after he returned to his boyhood club for a six-month spell last season.
The former Everton, Birmingham and Sunderland playmaker signed for Well in January in a bid to prove his fitness after three years of injury troubles but left McCall waiting while he weighed up his next move.
However, the Steelmen's boss says the 30-year-old, who scored a penalty in Motherwell's 4-2 defeat to Newcastle on Tuesday, is due to put pen to paper.
McCall said: "He will be in tomorrow to sign. I'm led to believe it's going to happen and, if it does, it will be another good addition to the squad. Once he's done it, it will be a shot in the arm for the fans."
McFadden appeared against Alan Pardew's men as a second-half trialist, but by the time of his arrival after the interval his team were already behind to goals from Haris Vuckic, Yoan Gouffran and Moussa Sissoko, with just Iain Vigurs' opportunistic strike to show for their first-period efforts.
McFadden soon reduced the deficit with a clever penalty but the youthful Magpies outfit again showed their class with a fourth from substitute Sylvain Marveaux as they wrapped up a comfortable win.
The home side started without new signing Stephen McManus but did include seven-time capped Estonian international Henri Anier as a trialist, and McCall said he would take another look at the striker in training.
The 22-year-old, registered with Norwegian side Viking FK, came close with an early shot over the bar, saw another effort blocked by keeper Robert Elliot after doing well to latch on to Simon Ramsden's launched ball over the top, and was then denied for a second time when his fierce shot was pushed away midway through the second half.
McCall said: "He could have scored in the first couple of minutes and then had another couple of decent efforts. He has got a good scoring record and we will look at him again over the next couple of days."