If you would like to make an extra principal repayment, you can do so. Simply enter a transaction using the ledger. This extra repayment of principal will be taken into account for the interest calculation that happens for the next periodic payment.
Examples of extra principal payments include (but are not necessarily limited to):
Contributing an extra $50 a month
Doubling the periodic principal repayment for every period. (The principal repayment can be found for any particular period by referring to the loan schedule.)
Making a 13th principal repayment every year. (This assumes a loan that is repaid in monthly installments.)
If you are doubling the principal repaid with every periodic payment, you will need to recalculate the loan schedule for each installment. This will allow there to be an accurate value for the required principal repayment with each installment.