Hello Heemu,
it´s no problem to add the miles after your flight. You can do it online on the thy.com homepage. This is what they say on the page:
"Claim Missing Miles
Inform us of your missing miles and make sure they will be registered to your account.
Only Turkish Airlines and Anadolu JET flights can be claimed online.
You need to send your original boarding passes and the ticket copies to Turkish Airlines Head Office 8th floor Product Development Management 34149 Yeşilköy, ISTANBUL TURKEY in order to earn mileage credit for flights flown on our Program Partner Airlines.
Retro claims are only allowed for activities that have taken place within the last 12 months."
As you already booked your ticket without having given in your miles&smiles number you can tell the agent at the check-in counter to type it in. They will do it.
Don´t forget to print out your miles&smiles membership card before. After your first registrated flight you will get a plastic card.