The loans your money is invested in are a contract between you and the borrower. A third-party administrator would take over the servicing of the loans. This administrator would also be an ISA manager so your investment would still be treated as an IFISA.
You wouldn't be able to add more funds into the Zopa IFISA and your borrower's repayments wouldn't be reinvested in more loans. As your borrowers repay you would be able to withdraw the funds or transfer them to another ISA provider.