If it a 26 TPI bottom bracket shell, then I'd punt. Trade it for a square-taper crank, find a new axle and use the original cups.
If you want to spend a couple of hours with no guarantee of success, then you could file down all but 3 good rows of threads on the BB cups- a 24 TPI cup will thread into a 26 TPI BB shell three turns. If you were to tap it to 24 TPI, those three threads would be all you get anyway- the others would be cut away by the tap. I'd leave the inner threads and file the outer (nearest the bearing).
The shell will be about 3 mm too wide, too- 71 mm rather than the desired 68 mm. You can file/grind this down as well, but it's mighty important to hollowtech bearings that the faces be parallel and tangential to the axis of threading. The crank may have 3 mm of tolerance there anyway, so I'd try it before grinding/ filing the shell. There is no such thing as a facing tool for 26 TPI BB's, which would be the right way to do it.