is there some kind of usb plug for a desktop?

Yes there are..

Wi-fi USB adapter, or Wi-fi internal wireless card.

yes. you can add usb ports, space permitting, in an available PCI slot. You can also add a wireless NIC (network card) to a PCI slot, if available. It takes no more than 10 minutes to do either one, device drivers included.

If your motherboard does not have onboard usb ports you can add them as mentioned above as long as the motherboard/bios supports it. If the board has what are called USB headers (places on the motherboard to plug in usb connectors), you can get a case with front usb ports, headset and mic jacks. For the latter two the board has to have the FP-Audio connector also. The motherboard manual will tell you where they are and the correct way to insert the connector. All three of those connectors are not like the ones on the back of the computer. You can insert them the wrong way.

Some motherboards like ASUS (and probably others) have Wifi built into the motherboard.

YES, just use a USB wifi adaptor, one shown below

http://www.ebuyer.com/product/161888

yes it can be compatible, like wireless.. no usb needed