Is it possible to add a co-borrower or co-signer when refinancing a mortgage loan?

Yes. It is possible to add a co-signer (or co-borrower) on a refinance.

However, the co-signer should be fully informed that they will be equally responsible for paying the loan if the primary borrower fails to pay and they should also have an ownership interest in the property so they won't end up paying for property they do not own.