Remember, by adding a resistor in parallel with the pot,
a lower total resistance will result. If you must keep the
total loading at 500k, you would start out with a new
pot at maybe 1megohm, then pad it down to 500k with
a large fixed resistance. For now, try a 1meg fixed
resistor (1/4 watt size is fine) tacked across the two
outside terminals of your 500k pot. It should broaden
the taper, but only load it down to about 300k resultant.
If it works well, and doesn't trim off excessive highs,
that's good.