I haven't seen the addendum but if it is simply advising of the change of ownership, and stating that the lease will be continuing under the new owner's name as the lessor, then depending on how your current lease reads, you may be expected to sign it. I am not privy to any of the information on the lease so I do not want to state an opinion based on the information on a standard lease form. I would advise you to speak with a real estate lawyer.
Currently in California, the short sale does not terminate the lease.The new owner taking title pursuant to the short sale becomes your new landlord and can decide to continue your tenancy or terminate it pursuant to the usual 30-day notice period, or a 60-day notice period if you have been a tenant for more than one year. Have you talked to the new owners, maybe they would want you to move out, in which case they may need to pay you "cash for keys" money to terminate the lease.