Amortisation schedule with fixed no. of regular payments, but includes one-off payments

Hi,

Can anyone assist with building a loan payment/amortisation table with a set number of repayment periods and with one-off scheduled repayments? i.e., the additional payments would reduce the amount of the regular payments, rather than reduce the no. of payments.

So the inputs would be:
- Amount borrowed
- Interest rate
- Loan period in months (i.e. fixed term)
- Start date of loan
- Amount of scheduled one-off repayment
- Date of scheduled one-off repayment


Thanks,
Brendan