If you have a bootable USB drive with an operating system on it, you can make a disk image of the drive. This disk image can then be used to install the operating system in a virtual machine. For information on creating the disk image and creating the virtual machine, see Creating a Fusion virtual machine with a disk image (1015406).
Note: Where that article refers to the Windows install disk, use your bootable USB drive instead. Ignore the note about a CD/DVD drive, as a USB drive is being used instead.
An added benefit of this method is that the disk image (saved to the Mac's hard drive) is faster to load and boot than the USB drive.
If you have a USB with a disk image on it, you can create a virtual machine and install the operating system directly from the USB drive. Follow the directions in the second half of Creating a Fusion virtual machine with a disk image (1015406), where the newly created .cdr is the disk image on the USB drive.