if i understand you correct, you want to use usb and found that it requires a 48mhz clock... the way to get this from other oscillator settings is to use the PLL configs. I believe the PLL derives a 96mhz clock from your (4..20.. whatever) input... then you devide that by two. this is done with the config flags and one define..

DEFINE OSC 48

Do a search on 'USBDemo' and study that a bit... it taught me quite alot and ive just started with it.

if i am wrong, i hope someone corrects me quickly

hope it helps