This is easily doable. The only hurdle you face is how/where you will spice in that power connection.
If you cut the orange plug off of the Kinect you will find that there are really only 5 wires you have to deal with. 4 of those are the standard colored USB (Red, Green, White/Yellow, and Black) lines and a Brown wire, which is your 12V lead. In short:
1) Grab USB cable you wont be using and cut off female end.
2) Cut off Orange plug and strip down to the colored wires, remove wire braid.
3) Match colors to proper colors, solder
4) Wire Brown lead to separate end for adapter
5) Wire additional ground to USB Ground (black) lead
6) Wire in new adapter end for power
Voila! There you have it.