You're going to have to mod the power supply. Snip the fan wires and run them to a rheostat (fanbus) to control the speed. I did it to mine. Pretty easy to do.
Here's what I did (mine has 2 fans):
1. Open psu, snip fan wires where they connect to the pcb.
2. Splice the fan wires together (black to black, red to red), and also have a long pair of wires to go out of the psu into the case. Solder these together.
3. Put the PSU back together, but make sure the long pair of wires go out of the psu with the other psu wires.
4. Attach a 3-pin fan connector to the long pair coming out of the psu. (may need to solder, only 2 connections of the 3-pin connector are used, make sure you get the + - correct)
5. Now connect it to your fanbus or other rheostat device and you should be good to go.