You definitely can. If you have a T5d-R or X5d-R all you need is the Qnexus TTL attachment. Our current PX receiver mounting brackets are compatible with Canon and Nikon, however, just using a couple pieces of velcro you can attach a PX receiver to the Qnexus.
What you'll need to do is locate the infrared “eye” on the back of your PX receiver (it looks like a little green circle) and using our velcro and foam mounting kit (available for $3.00 by contacting support) take the supplied foam ring and place it around the infrared eye on your PX receiver. Then take one side of the supplied velcro and adhere it to the back of the PX receiver and mate the other side to the Qnexus. Take note of the way you would need to arrange the unit to align the PX receiver's infrared eye with the sensor on the Qnexus.
Now you're good to go! Set up your gear like you normally would, and refer to the Qnexus manual for more help with setting up your Quantum gear. As some of the Quantum flashes are not high speed sync compatible, the PX system in turn will not be able to give that functionality to the Quantum flash. The PX system essentially “piggy backs” on your existing system to give you full radio triggering, TTL and that great extended range.