The USB module can work at 1.8V, but the problem is that the data lines at this voltage level are out of the USB electrical specification, so the USB host may not be able to communicate properly.
The electrical specification of USB is described in Table 7-7. DC Electrical Characteristics of the USB 2.0 specification and it can be downloaded from USB.org . The important parameters impacting this are:
Output Levels for Full-speed
High (driven) – VOH - 2.8V-3.6V
Input levels for Full-speed
High (driven) – VIH – 2.0V
This means that the USB receiver could operate at 2V minimum but the output high level should be between 2.8-3.6V to meet the specification.
Unfortunately, there is no way to separate the USB I/O voltages from the rest of the I/O pins without the use of an external level translator.