The axle thread on the Cassette version is only 12mm diameter, but I would expect the axle thread diameter of the freewheel version of the Magic Pie Edge to be 14mm, which is too big to allow the 12mm bi-hexagon shaped hole on the inside of the freehub body
(and its outer bearing) to fit over it:
Please note, the axle threads in the above drawing have been extended to illustrate my proposed modifications to the original design.
Check out
this thread for further information.
More details on the freehub unit and its bearings can be found
here.
Therefore, to convert your hub, I think you would have to replace the axle, nuts and washers, the R/H side cover and then obtain the correct threaded freehub unit to match.
Hopefully your freewheel equipped hub will be able to withstand the pull on the chain from the 500W middrive motor.
Alan