You can add as much extra data as you like in blocks of 2GB or 5GB at a time. If you are regularly purchasing extra data, it may be worth changing your monthly plan to a bigger one as it's better value for money. You can change your plan every month in the 'plan and add-ons' section of your account.
Please note that if you change your plan to a smaller one, your data bank will reset, and you will lose any of the data you had banked so far. From that point on, you can start banking any unused data again.