If you’re asking us here in HN, then it’s very unlikely that you have the sort of negotiating power required to get any sizable company to approve a customized set of legal documents relating to your hire. Changes to the standard set of hiring paperwork often require multiple managers and lawyers to sign off on it. For most positions and candidates, the effort isn’t considered worth it. You can certainly ask, but don…
I was able to add an addendum to my rental agreement created by a fairly sizeable property manager. Addendums are much easier for them to agree to.
Second, did the addendum directly contradict or void language in the standard form lease crafted by corporate legal? If not, there's probably more room for negotiation there.
Even in standard employee agreements, there's often an "invention disclosure form" that sort of operates as a built-in addendum for the new hire to fill out. This helps prevent the employer from claiming ownership over things the employee worked on prior to hire.