Depends on the bank, some have systems in place where you can add a top up payment and they will include it in the direct debit, with others you may have to do a separate standing order in addition to the direct debit and alternatively you may just be able to make transfers with online banking of varying amounts, it will just depend on the systems of the bank you are using.
There is no real catch to this if you have the spare cash, you will save interest, check if the extra money is reducing the amount the balance the interest is being calculated on just in case it is sitting as a credit on the account and not affecting the interest calculation. Obviously once you lodge the extra you can't get it back, some banks again may have a facility where the overpayment can be drawn down again should you need it.