Best of luck!
I agree with Randy, Todd and Lucinda. However, I partially disagree with Bob's answer. If you add a co-borrower to the loan and purchase contract, then the seller, regardless if they are private or a bank, will now have to be concerned with 2 borrowers qualifying instead of just one. Now, there could be double the risk of the borrowers not qualifying. I understand the sellers reason(s) for not allowing the addendum. But you need to know if you are REQUIRED to have a cosigner in order to purchase the home or is it just a suggestion from your lender.
Maybe you could offer to increase your earnest deposit and make it non-refundable in exchange for adding the co-signer. So, if you do cancel or can't get approved or can't buy it, at least the seller is compesentated for accepting the additional risk. Regardless of all this, what advice is your Buyer's Agent giving you?
Buyer's Agent Realtor