That listing agent is right- many, many REO properties, including Fannie Mae and HUD homes, do not allow changes to be made on their special addendums. Basically, as the seller, their stance is that if a purchaser doesn't like their terms, the next one will. And their terms and special addendums, when in conflict with our basic contract, supersede any conflicts.
Please note, it is not that the listing agent is trying to be difficult, they are just working for their client.
Hope this helps,
Krisan in KC