Basically, what you want is an L-pad. An L-pad is just a couple resistors which are carefully chosen so as to attenuate the signal, while keeping the impedance, or "load", in this case, the same. Here's an old chart I made showing how to do this when it was assumed that someone wanted to keep full output in the "split" mode, then attenuate the output for the full humbucker mode.
View attachment 52585
In your case, simply eliminate the switch, like this:
View attachment 52586
Make sense?
Artie
P.S. You'll have to select your resistors from standard values. Just get as close as you can to the "calculated" value.
P.S. 2: Just to expand on this a bit, imagine you want to cut your output in half. You're using a 500k pot. So R2 would be 500k, and R1 would be 250k. The 500k in parallel with the 500k pot will look like 250k. Add the 250k on top of that and the pickup is still "seeing" a 500k load. But now, the output (voltage) is divided by half, which is a -6db drop.