Well I don't think that would be possible. The only thing Q has been shown to be susceptible is the rulings and powers of those in command of the Q Continuum, so the only way the Borg would even be able to get him in a position in which he could be assimilated would be if he was deprived of his powers as he was in Deja Q, in which case he would essentially just be a human so it would be just as if another human had been assimilated.
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A cyborg would be one thing, but Q is far too much of a free spirit to want to be part of a collective as the Borg would require him to be. If he wanted to be a cyborg, I imagine he would just turn himself into one.