Addendum to an agreement

Once an agreement has expired, you can't revive it. In legal terms, it no longer exists. What you can do is create a new document with a new term supporting the expired agreement. And as mentioned the relations and duties were maintained between the parties even after expiration and the addendum extends or pertains to terms which survive termination or expiration, and is agreed to by both parties, then yes, it is possible. In some cases, the "addendum" is likely be more properly interpreted rather than a modification of the original one, but for the most part this is a distinction without a difference. Also, if both parties agree to it, the start of the new term can be backdated so that there is no period of time in which they are not covered by the contract.

Addendum is to add something to the present agreement. In your case as the agreement has already expired, an addendum wouldn't be the right thing rather you would need a renewal agreement.

Hello,

Your agreement for service is already expired. However parties have continued their relations adn decided to exten the relations for 2 years so instead of make addendum execute fresh agreemet stating all terms and condtions. 

Thank you all for sharing your views

Since your work is still continuing you may resort to the option of change order document for the agreement or the extended statement of work.  If both the parties agree then only we can prepare the change order to cover the time extended till the date the work gets terminated.  This can only happen if the work has continued without any gap.

Regards