You can always make additional payments by making a payment using your debit card or by connecting your banking account (VRP) in the Zable App.
Once you have a completed transaction on your account you will see the option to "Pay", from here you can make additional payments.
Please note that you wouldn't be able to make repayments towards a pending payment, however, once it has been completed you can do so in the repay section as stated above. Interest is not charged on pending transactions.