Yes. The MD-87 carries 3000 gallons of retardant internally. The "pod" is simply the aerodynamic fairing and door system to provide good drop performance.
http://www.eatanker.com/I forgot to note that Coulson is turning some 737-300s into Air Tankers as well, with 4000 gallon tanks and, because even then they're so far under MTOW when flying firefighting missions, they can leave the aft galley, lavs, and (I think) 30 seats installed at all times, making the airplanes very flexible since they can carry their crew and parts to the scene, or alternately transport a smokejump or ground firefighting crew to the Area of Operations and drop them at the base before filling up on retardant and heading to the fire, making it a uniquely flexible aircraft. Meanwhile, the DC-10s can carry in things like pumps, "inflatable" tanks, fill basins for helicopters, and the like to help set up a remote base that may not have all the equipment but does have a water supply.
I've not seen pictures of the interior of the MD-87s, but I suspect they have limited capacity to carry things. I know the 146/RJ-85 and C-130 birds have their interiors occupied by the tank system, so they are tankers only. But this is one of the other benefits of the jets - they can be more than just tankers.