Ppm (“parts per million”), refers to concentration of “something” in “something”, and is most often expressed in mg/kg or mg/l. In my opinion, you cannot convert ISO code to ppm unless you separate 4, 6, and 14 particles and determine their weight in a particular volume, and then calculate their concentration in a solution (oil in this case).
Anyway, why would you prefer cleanliness reported in ppm? How would it be possible to distinguish particle sizes, which is the primarily goal of ISO testing, from a single ppm account? I don't know how such reporting would be more practical and usable from the well-established ISO tables?