Unfortunately Virgin Mobile don't do "add on" data. Once your allowance is used up, if you continue to use mobile data you'll be charged at the daily rate of £2 per day for each day that you access it until your refresh.
With a Freestyle contract, if you're planning ahead and feel that you will require more data for next month you can change up your tariff and then back down again if your present tariff allowance normally suits.
If you want to say thanks > click 'Kudos'. Answered your question? > click 'Mark as Helpful Answer' also.