You can use to to 12GB of your allowance when roaming in the EU (assuming you're on a plan that includes more than 12GB of data). Does the message refer to your EU roaming allowance or your general data allowance?
If it's referring to your EU roaming allowance, you could buy a 1-day or 3-day EU data pass from within the Smarty app, which will give you access to your full data allowance for the selected number of days.
If it's referring to your general data allowance, I'm afraid it will not be possible to add any more data.
The unexpired data add-on you have is only for use in the UK, so can not be used in either scenario.
