Very hard to passively add treble. The best thing is to take steps not to remove it in the first place. So, you could use a 500k pot instead of the standard 250k which will get it sounding brighter. You could have a 500k(ish) resistor in parallel with a switch, which you could bring in to get back to the 250k sound. Installing a tone pot will also take off more treble. You could use a ‘no-load’ tone pot, which has a section of track removed so that set at 10, the pot is out of circuit, and below 9, it acts as normal. But that dies give a step in the tone. To avoid that, you could fit a Fender TBX tone circuit. This has a mid-position ‘normal’ tone setting with 250k resistance, but above that, a second stacked pot is brought in and you can wind the resistance up to 2 Meg, getting a sound like the no-load pot, but more controllable with no jump in tone.

Alternatives are
1) A resonant low-pass filter, but these tend to work more as a mid-boost than treble boost, and you do loose some upper treble in the process. You don’t get something extra for nothing with passive circuits, the energy has to come from elsewhere.

2) A high pass filter so you cut out some low frequencies. Neck pickups sound bassier because the amplitude of the fundamental and the low harmonics are far greater there than at the bridge position. The high harmonics are about the same level as at the bridge. So the tone difference is all about ratio of low to high harmonics. But whilst you can get a more bridge-like tone, you will reduce the volume as a result. So you could

3) Use an EQ pedal to cut the bass frequencies, possibly increase the treble, and boost the overall output.

4) Fit an active EQ circuit in the guitar.

But switching the pot value and no load/TBX tone pot changes can do a fair bit on their own and are relatively cheap to do.