Some WRT54 models have USB ports on them. If yours already has a USB port, then you can install CUPS into DD-WRT and connect the printer, and it will (probably) work. If it doesn't have a USB port, there may be a way to add one, depending on your model.
If not, you'll need a separate piece of hardware. As others have pointed out in comments, a USB to Ethernet converter will not work - it adds a USB NIC to a host, rather than letting a USB device talk over ethernet.
One other thing to be wary of is that your printer may not work with CUPS. Many cheaper USB printers require windows-only drivers, and if yours is in this list then you may be out of luck. Might pay to check that it's supported before going further, eg at www.openprinting.org or at www.cups.org