You can't as there is no filter thread and front lens element is not flat but convex and protrudes out conspicuously. Also, the angle of view at 7mm is so wide that any filter would have to be ridiculously big in order not to vignet.
Anyway, according to Ken Rockwell, the use of polarisers is to be avoided for ultra-wide lenses because of the weird effects they created. This is because the polarization of the sky varies from one end of the sky to the other. It is not a problem with a tele lens but the angle of view is so wide with a ultra-wide and covers such a broad expanse that the polarizing effect will vary greatly across the sky, so much so that it is possible to get a dark area in the sky in one part of the image and light area in another.
Well, I have no problem using a circular polariser at 11mm on the ZD 11-22mm, so I would guess there could be problem only near the 7mm end of the ZD 7-14mm.
Maybe the Cokin X-Pro filters could be used on the ZD 7-14mm but the cost would be prohibitive. The cost (from B&H Photo) of fixing just a X-Pro Circular Polariser onto the ZD 7-14mm would comprise:
(a) X-Pro Filter Holder: US$54.95
(b) X-Pro Universal Adapter Ring: US$66.95
(c) X-Pro Circular Polariser: US$330.95 !!!
Total: US$452.85 before air freight & GST
The Cokin X-Pro Universal Adapter is used for those lenses (up to 118mm) with non-standard threads or damaged threads and is attached to the lens body itself. In the case of the 7-14mm, it might be possible to attach to the built-in lens hood.