So you need a repeater, that's the fundamental issue here?
While you can use the Alfa as a repeater, either using its built-in hotspot feature, or using Connectify (as long as you have the paid version), it always requires the laptop to establish its USB connection. There's no way to hang the USB cable off a router.
If you want a better solution, by all means, use a standalone device dedicated to that purpose. Some routers support repeating as a configurable feature, while others support repeating via third party firmware (e.g., dd-wrt). Fortunately, at least according to the dd-wrt database, it appears the WNDR3400 router is supported.http://www.dd-wrt.com/site/support/router-database
But again, it has nothing to do w/ the Alfa USB wireless adapter. The dd-wrt compatible router itself does all the repeating. So the fact the router may or may not have a USB port is irrelevant.