Lock a HELOC at end of maturity to a 10 yr fixed balloon term with a 15 yr amort.
Fixed rate and closed-end are often used interchangeably on these threads which causes confusion since they are not one in the same.
You cannot convert the open-end credit to a closed-end credit without providing all the necessary closed end credit disclosures required by both Reg. Z and RESPA.
If you are leaving the HELOC as an open-end credit and require the mentioned repayment terms during the repayment period then I suggest you contact your software provider for assistance in how to set up such terms in your software. If the software cannot handle those terms then you will need to rethink how you want to handle the repayment period.