After a bit of web searching, I can do you some info on how phantom powering works if you want, for a rather longer answer.

The short answer is: You should be able to use both provided you put diodes in so that the diodes will automatically select whichever voltage source is active - for this, the phantom power supply voltage must be at least 0.7V more than the battery voltage otherwise the battery diode will stay partly turned on. The diode in the battery line will reduce the voltage by 0.7V, which may affect your pickup's performance, I don't know. To get phantom power in there at all, you'll need to (a) change your guitar's mono jack socket to a stereo jack socket or XLR socket so that there's 3 connections available, (b) get a suitable 3-way lead to connect it up with, (c) get some electronics to convert the low-current 48V phantom power supply into a higher-current lower-voltage supply which is more pickup-friendly, (d) hook this electronics somewhere in the guitar.

Then you need your mixing desk to be able to supply phantom power. I think this is often an optional extra. If so, you can make up the circuit easily enough - it's just a 48V supply and a couple of resistors, so the only complicated part is finding a cheap 48V supply (note that this supply must be DC, and must be regulated).

Are you sure you mean D-cells?! Really? How many? Doesn't that add significantly to the weight and balance of the guitar? One alternative to the might be to cut a (very neat!) hole in the guitar someone unobtrusive and fit a battery holder - these are available from most electronics catalogues. Most pickups/preamps these days use a single PP3 9V battery, but if yours uses D-cells then you can get a holder which takes the same number of AA or AAA cells, since they're the same voltage as D-cells. They won't last as long as D-cells, but if the holder's easily accessible then that's not a problem, right?

You may not think so, but that's the short answer. The long answer will only appear if you _really_ want it! :-)

Graham.