The DDK3 consists of a PBM30 amp, SPC15RP horn, SFR12 strobe & PSW5 power supply. The SPC15RP is an 8ohm horn if you were to add additional speakers you will have impedance issues.
You would be better off purchasing a PBM30 amp max wattage output 30 watts and a 70 volt horn such as a SPC30RT that has multiple wattage taps (30w,15w,10w,5w, 3w, 1w). As long as you do not exceed the amps max wattage minus a 20% buffer you could add more horns/speakers.
You could also add any size amp you like to fill your requirement