I got 2 Xbox wireless controllers for my PC along with a wireless receiver adapter. I'm considering getting some more, but I'm not sure if it's a good idea now that the Xbox one and its controller are out (meaning the 360 controller will be phased out soon and games might use a new XInput api that's incompatible with the 360 controller)
So I was wondering if it's possible to connect an Xbox one controller to the xbox 360 wireless receiver for PCs.
No, you cannot connect the Xbox One controller to the 360 receivers. They are different controllers with different tech and mechanical workings inside, so they share 0 compatibility. You can, however, just plug the XB1 controller in using any old micro USB cable (they are common household items nowadays) and it will work.
As others have stated, you cannot connect an Xbox One controller through the Xbox 360 wireless receiver. You can buy the Microsoft Xbox One Controller + Wireless Adapter for Windows 10 ($62.99 USD) that includes the adapter you will need. A better option however would be the bluetooth model as it does not require the rather bulky wireless dongle. This particular model was introduced with the Xbox One S. Pay close attention though, only these specific models of controllers have the universal internal bluetooth adapter. These controllers can be identified by the front face plate which is one solid piece of plastic as seen on the left in this figure:
If you don't have Windows 10, or are experiencing connection issues, refer to my post here where I posted a work-around for such problems. You can purchase the Xbox One Wireless Controller here for $59.99 USD.
Note: that Microsoft no longer sells the model that includes the dongle from their marketplace.
As of now, no. As for the future, very unlikely. Both controllers use a different protocol to communicate with their respective receivers. While it might theoretically be possible, it would need to be done on the receiver side.