I don't know why a CPL holder would want to go the MPL route. Any airline that accepts the MPL would likely accept a CPL/IR for a normal training program...
I don't see the advantage of jumping into a "partial MPL course," even if such a thing existed. An airline would be better off getting the candidate through the remainder of the IR so he could then become a candidate for Captain in a few years. It would be better for the pilot, too!
In either case the airline would make the decision whether it makes the candidate pay for the training. If an airline paid for an MPL course, then it might allow equivalent cost credit for an IR and normal sim training for new FOs. If the airline required the candidate to pay, I see NO advantage for a CPL holder to go the MPL route -- especially if he has already been through the ATPL courses and tests!