No, you can't, which is one major reason that I fly AA instead of UA. AA allows you almost total flexibility with vouchers, adding them up however you want to, while UA issues you a booklet (in this case, it would be a booklet of 4 vouchers in $75 increments) and allows only the combination of THOSE 4 vouchers...so if you're buying a ticket that's more than $300 you will pay cash for the rest of it. Also, if I remember correctly, if your ticket were, say, $100, you would use two vouchers and get NO residual amount back in another voucher. I could be wrong on that last part though.