I did one similar to this with a SBA loan. You are right that it is a challenge, but it can be done. If you can not find another option, I would suggest contacting their current SBA lender. For one, they are the most likely to approve the loan since they already have the first. The second reason is that even if you had another SBA lender that was willing to do it, the first SBA lender essentially has a first right of refusal. The below quote is pulled directly from the SBA SOP:

"a.Procedure to refinance an SBA-guaranteed loan:
i. Borrower or Lender must provide evidence from the lender holding the existing SBA-guaranteed loan that verifies the lender has declined to approve an increase in loan amount or a second loan and the lender is either unwilling or unable to modify the current payment schedule. Lender must retain this evidence in the loan file".

So they either approve the loan, or give you a letter stating that they are not willing to extend additional credit to the borrower. Once you have that, then you can contact a different SBA lender, and then they can refinance the current SBA loan, and get the additional loan for the borrower. You will hear many people say that you can't refinance SBA debt with SBA debt, but that is simply not true. If you do a google search and download the SOP it clearly states in there that you can, you just have to follow the rules (for example another threshold is a 10% savings in the new payment).

I don't contribute to this thread much because I am not a MCA expert, but when it comes to SBA thats pretty much all that I do. Best of luck!.