Adding a filter (or a filter and Q switch) is probably best achieved by taking the existing electronics out and using them in another project and replacing with Signature electronics. You can keep the bass and treble switches in this bass and have them either dedicated to one pickup or the overall sound.
Working around the existing holes, something like this could function:
The dotted line represents a small undercut you might need to make to accommodate the extra Q switch and make the layout look like it wasn't an afterthought. The green dot is where I'd put the volume control. I'd use the existing volume location for the new filter, and leave the pan and original filter locations the same.