Before starting a driver conversion, you should have read the general instructions at Converting drivers to the new wifi stack. Many items there are marked as "not for usbwifi" though, so instead of a big list with lots of things to skip here is a todo list when converting a usb wifi driver from the old stack to usbwifi and the new stack.

Note that the usbwifi(9) manual is only available in the hg branch sources!

Similar to usbnet(9) the usb wifi driver to usbwifi(9) conversion is mostly an excercise in cleanup and deleting. Here is what is needed to be done:

Look at one of the converted usb drivers on the branch (see ?Driver state matrix for a list of usbwifi(9) drivers and their state.)