I've done a few custom wirings for stingrays that include a passive option over the years. There's several hurdles that even fairly experienced guitar techs might balk at. You'll have to find someone who knows what they are doing.
A stingray is not a typical active bass in that it has the volume pot wired to the output of the pre instead of at the input. Relocating the volume pot to the front of the preamp is problematic with MMs. The 2-band pre has a very low input impedance (80-100K IIRC), and the 3-band has a very capacitive input which creates that signature clanky treble tone, and having a vol pot in front of this will effect that.
So you'll have to think about what you want from the passive setting. Is it just a way of getting through the gig in case you get a flat battery, or broken battery snap? In which case, you could have a switch that bypasses the whole preamp and takes the pickup signal straight to the output jack. If you needed a volume control, you could use the existing (25K) volume pot in this scenario, but even with the parallel-wiring/low-impedance/low-inductance of the typical stingray pickup, a 25K volume pot will really dull the tone and reduce the output slightly.
If you want a full-featured set up with passive volume and tone, then realistically, you'll have to mod a dual gang volume pot to have both low and high resistances. This way, you can switch to a 100k or 250K volume pot resistance for passive mode. It's actually not as crazy as it sounds. I have done this on several basses. (In fact, my main gigging bass is wired like this.) You'll also have to think about stacking say, treble and bass pots so you can have a spare hole for your passive tone control.
The other hurdle is with the existing pots usually being soldered directly to a circuit board, and in turn being used to mount the preamp. Your tech will have to be comfortable changing to custom pots in this situation and making them work.
You'd probably also have to think about installing a toggle switch since push-pull pots will probably be too deep to fit between the circuit board and the control plate...
