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不可能事 is in GG5, etc, as "an impossibility". 不可能性 is fine - it's more the quality of being  impossible.

You have the following on the site for an imposibility, but not the above:-

不可能事 【ふかのうじ】 (n) an impossibility.

Whilst an impossible thing may be "an impossibility," it seems to me that "impossibility" itself should be represented by this submission. The difference in favour of this submission for the concept of "impossibility" and the above entry on Google speaks for itself. I am not stating that you should not have both entries, but certainly you should have the above submission if it is a matter of choice.

Kenkyusha also gives this submission for "impossibility".

You also have the following on the site:-

不可能性 【ふかのうせい】 impossibility; WI1

And, of course, you have the entry for "possibility".