I highly doubt that there is such a thing. The entire point of the wireless keyboards is that you have
no cord going from the keyboard. There would be no point in someone creating a wireless transmitter if you still had to plug a wired keyboard into it. This is a rather unique problem, don't expect to use consumer products.

BTW, keyboard hacks aren't that hard. Plus you get way more inputs than most encoders. The only problem is if you have already wired all your buttons, but that is still not that hard.