You can use low / high pass filters to shape your sound, but you do need to design them properly...
If you create a filter using just passive components (capacitors/resistors/inductors) you will be able to shape the output response of your pickup, the only tricky part is figuring out which frequencies to boost and to cut. The other thing with just using passive components is that you need to know what the frequency response curve should look like, because there really isn't much variance in components, and once the circuit is made you can't really change it easily. The other thing to remember is that adding components to your guitar will increase the amount of noise you get in your signal, but probably not too much. So you can do a couple of things:
1)get an active filter board from someone like EMG and install it on that pickup (should fit in the control cavity, runs of a battery) that should allow you to boost as well as cut frequencies and to twiddle about a bit (which is good, can't really do that with just passive components)
2)use resistors/capacitors/inductors that are tunable (they normally have some kind of screw on them that will allow you to tweak the value) and build a circuit. You must however remember that a pickup is an inductor, and that that will also shape the response of your circuit. If you take this route then it might be an idea to run your guitar into some clean source thru an eq unit, so you can get an idea of which frequencies to boost/cut. Then when you've got an approximate curve you can set about designing your filter. If you want some help on this, then I am sure I can give it a pop (i supposedly did an electronic engineering degree)
Best way in my opinion is to get that active filter board, at least that way you can change and tweak it quite easily if you are not happy later down the line, but then again, you do end up wondering where your sound has gone every once in a while when the battery runs dead...
just my thoughts... Hope you find a solution to the problem!