Once you accepted your SBA loan and signed the closing documents you can not change that loans application.
Your Loan Officer will be able to process a second loan from the SBA using the manual forms process.
You should call and email your loan officer and ask them to process another loan in the amount you need. This may actually be greater than the original loan amount or about the same. Be sure you know what you need and are ready to ask for the total amount.
If you have not closed on your SBA loan you can ask your loan officer once they make contact to modify the current loan application.