I have a similar question to that which was posed in this forum post but was never directly answered:
If I'm using the USB DP/DM pins on the K66 (pins 19 and 20) but don't need the K66's USB regulator (VREG), can I just tie the VREG_INx pins to my schematic's externally regulated +3.3V supply along with VREG_OUT? The datasheet specifies what to do with the pins if the USB function isn't used at all, but it's unclear what to do with them if I want to use USB communication but don't want to use the internal USB regulator.
Yes, I think it is no problem. Figure 53-1 in RM shows that VREGOUT connects USB-FS and USB-HS. And When you add 3.3V to VREGIN0, it will work in pass-through mode. I test your idea on a TWR-K65 board. I disconnect J23, connect J23-2 to 3.3v and connect J9-7 with J9-8. It works fine.