This is exactly what I do. I have an amendment on my contract about intellectual property being owned by the company changed from "while under employment" to "using company equipment in employed hours". They sent me two copies they had signed, I modified both with the change, signed it and scanned the contract.

The returned copy was accepted by HR via email (not sure if they ever read it or not)

What will be interesting is if this ever gets to be tested as not ever sheet in the contract was signed.