Possible to convert dead 18v battery into wall-plug?
Get a 18 V power supply that is rated at least for the current that the batteries were able to put out (a couple amps minimum, probably).  Gut the battery module you have and connect the 18 V wires from the power supply to the battery connector internally.  Be really careful to make sure the +18 V goes to where the battery + went, and the power supply return goes to where the battery - went.

There is a off chance that it still won't work if this was a "smart" battery module.  In that case, there will be some communication going on between the drill and the module.  However, this is unlikely in a volume product like this where consumers largely buy on price.