You shouldn't need to override. Assuming the interest is US Treasury Obligation you would move the amount from a box 1 entry to a box 3 amount. However, the brokerage should have already put the correct amount of interest earned from US Treasury obligations in box 3.
Just because an entity does issue interest that is exempt doesn't mean it all is. Below is an extract from MN Law on exempt interest subtractions. So while Federal Home Loan Bank and Federal Farm Credit do issue exempt interest not all of their obligations are US Treasury and necessarily exempt. The brokerage likely has included the correct amount in box 3 already.
"Even though a listed agency generally only issues either exempt obligations or taxable obligations, it does not follow that each and every obligation carrying the name of that particular agency is either exempt or taxable. An agency may issue its own obligations that are exempt and also may handle private obligations that are not exempt. For example, the agency may administer, purchase and sell, insure, or guarantee an otherwise private obligation. Such action by the agency does not convert a private obligation into a direct and primary obligation of the United States of America and, therefore, does not make the private obligation tax exempt. The taxable status of each obligation must be determined separately..."
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