Hey: If I understand what you are trying to do, it won't work. The mini usb on the dock that connects to the phone does not carry the audio signal, only 5.3 volts and the data signal. See this page for the pinouts:
http://tinyurl.com/ytufmh
What you could do is to is to rip the mini usb connector out of the dock and replace it with the cable from the dongle. I did something along these lines and replaced a mini usb with the 11 pin usb connector. I got some epoxy putty at Pepboys to hold the connector in place.