Is the RC4 KSA the same as an extractor?
It would not work as a randomness extractor as there is a detectable correlation between the key, the internal state, and the output.

http://saluc.engr.uconn.edu/refs/stream_cipher/fluhrer01weaknessRC4.pdf
https://eprint.iacr.org/2011/448.pdf

It does expand the key by concatenating it until it fills a 256-byte array and it is used to mix the internal state. So it acts like it, but it is not one, and should not be used as one.