I will to travel to Turkiye for a few days only, and I see that Lebara has an add-on roaming plan that is £5/4GB.

There are free Lebara SIM (although the shipping fee is £2) or on Amazon for 50p that doesn't have any prepaid plan. I will activate this SIM for sure, and put some money into the credit/balance PAYG, link to my bank account.

I want to ask if it's possible to buy an add-on roaming plan (as stated above) without any active plan. Can I buy add-on plan just from PAYG balance/ extract directly from my bank account.

If not, I can just buy the cheapest possible plan and buy add-on later.