Is subclassing a valid verb?
Solution 1:
I have been subclassing for so long it had better be a verb.
Seriously, nouns can be easily verbed in English. We photocopy things, basketball players brick a shot, we table discussions. And on and on.
Solution 2:
To subclass became a verb when Bjarne Sjoustrup, the creator of C++, coined it. He being of worldwide reputation, and using the noun as a verb, it instantly caught on.