You've got an adapter/cable usb-A -> mini-usb-B, i guess... You have to modify this adapter and connect pin 4 to pin 5 (on the side of the mini-usb jack), see wikipedia for the pin-out. But I've got no guess where to buy such a cable/adapter. Maybe the easiest way is to build your own adapter.
Your idea of buying 2 additional MD-80 is also great

But I prefer using all of them in parallel to get the optimal result

I also mentioned another idea in two other threads. You could use an external li-ion battery instead of the internal. But you have to open the MD-80 then.Then you're able to connect it to an external 1xAA battery holder and use trustfire 14500 batteries (size AA, but li-ion! 3,6V) you only have to replace the battery every 3 or 3,5h (maybe SD-Card too). It's much cheaper then buying several MD-80s, but you have to open the MD-80 and solder a little bit.
I've tested my MD-80 with another external battery (from my digital camera) and it worked for almost 14 hours. The only problem is you have to replace the SD-card every 5h
