Admittedly, the reikna-pyopencl interop is not very well documented at the moment. What you need to do is:
1. Create a reikna Thread object from whatever pyopencl queue you want it to use (probably the one associated with the arrays you want to pass to FFT)
2. Create an FFT computation based on this Thread
3. Pass your pyopencl arrays to it without any conversion.
(you can create a reikna array based on the buffer from a pyopencl array, by passing it as `base_data` keyword, but if using FFT is all you need, that is not necessary).
Reikna threads are wrappers on top of pyopencl context + queue, and reikna arrays are subclasses of pyopencl arrays, so the interop should be pretty simple. Please tell me if you have any issues. I'll look at your SO question too, but that may be harder to answer.
Incidentally, I'm aware of all this baggage and weird design decisions I made :) I'm now in the process of separating cluda from high-level stuff (computations/transformations/etc), and adding a more consistent PyOpenCL/PyCUDA interop, automatic multi-GPU support and other features. I'm expecting to have a publish-ready version next month (high-level reikna needs a big redesign too, but that's the next step).