This is not recommended, not practical, and I don't think it's possible.
USB Storage Devices store memory as Flash memory, which is different than the type of memory used in your RAM. They are accessed differently and have different characteristics.
The main difference is that the Flash Drive retains its memory when the power is turned off, whereas the RAM is completely erased when you turn off the power.
Also, if you manage to get the USB Drive working as RAM, it would be like driving a car without circular wheels. It is just too slow to be used as RAM.
Can you not purchase any more RAM? It doesn't come too expensive.