You cannot plug in a regular PC USB wifi dongle. Belkin makes a NAS USB hub
that you can plug into your wireless router and connect up to 5 USB devices (hard drives, printers, etc). It's a bit pricey though. It's actually cheaper to replace your current wireless router with one that includes USB NAS support like this one
Another option is the Dlink DSM-G600
. It's an external wireless NAS enclosure that is made for older PATA hard drives, but it also has 2 USB ports for attaching additional external drives. If your current external enclosure has a PATA drive inside, you can take it out and stick it in the G600. If not, you can plug it in over USB. It's no longer manufactured, but shops that still have stock usually sell them for cheap.