how to make a W potentiometer from a log or linear pot?
What are you going to do with the result?  Things are mostly controlled digitally nowadays, which is why there is little point to non-linear pots anymore.  Set up the pot to drive the A/D input of a microcontroller, then perform whatever non-linearities you want on the resulting linear reading.

Even in the unusual case where you really do want a non-linear analog voltage from the user setting, you can still use a cheap micro (under $.50) to read the pot, perform the non-linear function, produce PWM from than, then a R-C filter to make the average voltage level.  The linear pot plus micro is often cheaper than the fancy low-volume non-linear pot.