Another option to consider is the DTX Multi-12 which would be a good compromise for connecting to your DD-65 and providing extra pads (plus an external 3-zone snare pad). The M-12 has Midi In & Out and can be connected to your computer to trigger software drum sounds. As Wildbill explained you will have to connect the Midi Out of one unit (DD-65 or Multi-12) to the Midi In of the other and add a USB-Midi interface to the computer. For some useful midi info check page 30 of the DD-65 manual and this link: http://www.midi.org/aboutmidi/intromidi.pdf
The preset drum kits of the Multi-12 can be tweaked to sound better (with 200 user kits), and to mix its audio output with the DD-65's just connect Phones Out of one unit to the Aux In of the other and feed it's output to your amp or headphones.
Good luck with your module choice.