It’s common to receive a mail from some service that they’re changing the terms and conditions of their service (internet provider, gmail, etc.) These changes are unilateral and can be about anything, from their privacy policy to the money they intend to charge me.
Why can’t I unilaterally send them a mail and say, I am changing the terms of my patronage and will now insert my new policy (pay only xx amount, or demand a mint Chico chip ice cream every Tuesday)?
You can. But the prefilled one implies they’re already OK with the terms. Adding/changing stuff requires their approval. If both parties agree to the altered version, then there’s no problem.
I doubt they’ll allow a special case just for you, though.
I have returned additions/corrections to employment contracts and had them accepted. My usually go to is to change the overly broad “we own all IPRs developed by you” to “we own all IPRs that come from work related tasks or as a direct furtherance thereof”. If they want to own anything I developed outside work for my own projects then that’s fine, but then they’d have to pay extra.