Probably not much of a market for that I suspect, standard rechargable batteries are just to easy to get and the cost is too low for folks to make much money selling a battery pack. You may be able to get one of the battery sellers to custom make you one if you really want one (and are willing to pay for it), but it'll probably just be some standard AA sized cells packaged into a set and won't offer you much over doing it yourself using some retail NiMH AA cells. Do a Google search on "Custom Battery Packs" for who can do this.
The issue with a drop-in charger is there are no external contacts for the charger to use (besides the radio's external power connection that isn't "drop-in" friendly). It may be possible for you to do a custom mod on your radio to accomplish this, but you'd have to address issues like shorting out the batteries when you set the radio down on something metal.