You can.
But before you put it to sleep, you'll need to put the USB module in suspend mode UCON.1, which can only be done reliably when unplugged.

This disables the USB voltage regulator and stops the clock to the SIE, which will give the lowest current consumption.

The USB routines are not setup to handle a resume.
So your best bet it probably to connect the +5V from the USB cable to one of the INT pins with a pull-down resistor.

When the cable gets plugged in, it will generate an interrupt, at which point you would execute an @ RESET to reset the processor and initialize the USB again.

HTH