Lots of good repair shops can help with that. But be careful -- I don't mean the service guy at Guitar Center - I'd look around for smaller music stores that deal with professionals and then start asking around (a lot). That's how I found the guy I worked with for years, doing exactly the kind of projects you described.
Once you find your techie, be sure to get clear estimates on how much time and cost to expect for the work - it can wind up more than you expect if not careful. Finally, from painful, personal experience: If you are having a preamp installed, get the unit prewired (basically, harness built with pots attached) from the factory if at all possible. My guy and I had a nightmare once with an Aguilar OBP3 that was not prewired (it only costs a few bucks to buy it already wired up). Great preamp (one of my fave's) but too many color-coded wires to believe.
Good luck - it's fun to spec out your own bass. Do the parts you are prepared to do, and don't choose anything exotic you've never heard or played (sometimes results in expensive disappointments).