Can air be ionized using microwaves? I've read about the concept of a laser induced plasma channel where air can be ionized to form a plasma using laser light.  Can this be done or is it hypothetically possible to ionize air with microwave radiation?  I know that microwaves don't have enough energy in electron volts to cause air to become ionized but what if the microwaves are generated with a high voltage?  Can these microwaves have enough energy to cause ionization? 
 A: Yes, you can achieve air discharge in the atmosphere using high power microwave radiation. The mechanism is avalanche ionization (https://apps.dtic.mil/dtic/tr/fulltext/u2/a172227.pdf)
A: 
Can these microwaves have enough energy to cause ionization?

No.
As you yourself pointed out, the energy level of photon in the microwave frequency range simply does not have enough energy to ionize atoms and molecules.
As for voltage, a magnetron already uses several thousand volts to generate microwaves. Household microwave ovens use a transformer to step up 120 vac.
UPDATE:
This update in relation to the answer given by @akhmeteli. I'm not disagreeing with the answer but feel it is very important to stress that the ionization of air referred to is due to high voltage, and not the magnitude of the electromagnetic energy of a microwave. 
The reason I want to stress this is because there is already much confusion on the part of people regarding ionizing vs non-ionizing radiation. Microwave radiation is non ionizing radiation. I don't want readers to be left with the impression that microwave radiation is ionizing radiation.
Hope this helps. 
A: Microwaves at typical strengths produced by typical equipment can not ionize individual neutral atoms in the air the same way that individual UV or X-ray photons can. 
This is because the energy of individual photons at microwave frequencies are way lower than the ionization potential of atoms. 
The two caveats are 


*

*They can get a hold of a random free electron in the air (of which there are always some) and slam it into other atoms and cause more ionization if the power is high enough and lead to plenty of ionization or even breakdown as described in this answer.

*If the power is really really absurdly high, then microwaves can certainly field-ionize neutral atoms the same way a high DC electric field can. However this would be on the order of volts per Angstrom. For DC fields this is done with sharp needles, for microwaves in free space you'd need an extremely high power field to get the order of a volt per Angstrom. However you could use the needle again (similarly to what's described in this answer), or perhaps even a fork.

A: Rather than revise my previous answer (as it has been accepted) this will expand upon it.
I believe the confusion as to whether or not “air can be ionized using microwaves” is  due to  confusion between cause and association.
Microwave radiation can be associated with the ionization of air. But the ionization of air is not caused by the interaction of microwave radiation with air.
An example of association  is the arcing that can occur in a microwave oven when metal objects are placed in the oven not designed for use with it. Induced high voltages cause the air to ionize and break down. This is a high voltage phenomenon. This association does not mean microwave radiation is ionizing radiation.
The ionization potential of air is about 14 ev. The energy of a 2450 MHz microwave photon is only about $10^{-5}$ ev, far below that needed to ionize air.
Hope this helps 
A: If you put a metal object in a microwave oven and switch on the power you will see sparks, hence microwaves can cause ionisation of air. 
A: A better question would be: can a maser (M.A.S.E.R. = Microwave Amplification by Stimulated Emission of Radiation; very much like a laser, but using microwaves instead of light) of sufficient power produce air ionization (blooming) along its beam path?
