Add a 2nd (passive?) pickup to a Sterling ?

Hi folks, really appreciate any input on this as I'm not sure which way to jump...

I plan to add a P-type pickup to my MM Sterling (it's a USA-made SUB type - the old sort)
I know I'll need to flip the orientation of the P pickup (a la Sandberg) to get it to fit - that's fine, I can chisel-rout and have found someone to make me a pickguard.

But what about the electrics?
The Sterling is active, with volume and 3 band eq.
I like the passive P sound, but have no aversion to adding an active Pickup as long as it retains something of the passive P sound (not looking for perfection, just tonal options in one instrument)

If I add a passive P pickup, how might I incorporate it in to the controls? I'm guessing that the pots in the Sterling are appropriate values for an active pickup, not a passive, right?

In one scenario, I might be happy changing the preamp in the Sterling to a 2-band, leaving a knob free to act as a volume for the additional pickup

In another, I could add a stacked pot and have the whole thing going though the existing preamp ?

In a perfect world I'd have a passive setup for the P pickup and the current active stuff on the MM one. Is that remotely possible?

I'm throwing ideas out - but if this was yours, how would you go about it?

thanks in advance
Tim