Roaming in Turkey

It is my intention to buy the 8 day plan and add it to my sim only,as we are only in istanbul for 2nights. I have been told to buy and activate here in the UK, but if I do then usage starts immediately, but I wont be in Turkey for 14 days so the plan will have expired.Can I buy the add on in Athens or Rome before I get to Turkey

A lot of travellers who travel use EsimDB if phone supports esim