Has anyone else added USB plugs on their xbox and using Linux?

Yes.

Quite simple if you are OK using a soldering iron. Not so simple otherwise.

2 methods, one involves opening the box.

First

Buy a cheapo USB extension cable. Cut the male end off and save for future projects (Think 5VDC supply on your desktop for...) cut ~2 inches of insulation off the cut end of the cable exposing the 4 wires inside. Strip ~1mm of each wire and tin the ends (heat up and melt a small amount of solder into the wire.

Now for the hardpart:
1) open your xbox and note the wires connected to the xbox game pad sockets on the front of the box. The colours are the same with the addition of a yellow wire.

Cut the wires and remove the two rightmost sockets (Cut the wires with enough length on both sides to solder to).

Match up the colours and solder away.

2) cut the cable off an old controller and solder the wires on, or just strip back the insulation on the wire and connect the USB socket to the game pad wires.

It is actually much easier to open the box.

Have fun.

there is an easyer way to add USB ports check out this mod chip http://www.mrmodchips its a lil pricy but should work excellent and look so much better