I bought an ATK pickup on ebay 4-5 years back, been meaning to do the same thing to my 400 (or another ATK).
Not sure which value to use, the higher the value the clearer the shift between pickups I'd imagine. Not sure if you have to be particular about it being active or not, or if you can just blend them using whatever you fancy prior to the preamp.
Some of the new ATKs have a J pickup at the neck position which is controlled with a blend pot (they used to just use a selector switch). It may be worth seeing if you can find what value of blend they use?