Ainsworth, 26, has scored four goals in his last four games for Well but was due to return to the Millers on January 7 after making his initial loan switch in August.
However, the deal has been extended following talks between the two clubs and McCall is now hoping to persuade Norwegian club Viking Stavanger to let him keep hold of Estonian striker Henri Anier.
Anier has contributed seven goals in 23 appearances for Well but is set to depart later this month.
McCall said: "Our chief executive Leeann Dempster and the board are working as hard as they can to find some creative ways that they can find some money but at the moment there is nothing to report.
"If it was just a straight forward matter of taking him on a short loan again it wouldn't be a problem. But I think Viking are looking for us to take him longer. At the moment we are only paying half his wages but if we do take him longer we would need to pay all of his wages.
"But we remain hopeful. Henri is desperate to stay. I had a good chat with him and he says he is enjoying his time here and feels settled."