[NAME] I do not think there are any USB headers on the motherboard. To be sure, you should check check the online XPS 8950 Service Manual . Dell XPS motherboards do not come with extra fan or USB headers.
I am adding a Corsair Fan controller but I dont see any available USB header. Anyone know if its there? alternative is to convert to USB-A and route outside the case. Ghetto!   I got lots of fans! 8 in total 2x 140m Corsair AF140 intake 3x 80m side intake 2x 120m Corsair ML120 on a Vetroo V5 CPU Cooler 1x 120m Swiftech on the rear Exhaust 8-way PCI slot Fan Controller with "volume knobs"  
XPS 8950, where are the motherboard USB Headers?