You can use Asiana miles to book flights on any *A airline, including Turkish Airlines. And because Etihad happens to be a non-Star Alliance partner of Asiana, you can actually use Asiana miles to book flights on Etihad too. You can't convert your Asiana miles to another airline's miles, and there is no need to do that anyway, when you can use the Asiana miles to book flights on other airlines.

As an example, in the past I have used my Asiana miles to book flights which were flown by EVA Air and United.

You must redeem your miles through the Asiana Club portal (see here), or by calling the Asiana reservations department. For Etihad flights, you will have to call them, but for flights on *A airlines, you can usually do the booking through the Asiana Club website yourself.

Of course, you firstly need to find seats available to book with miles on the dates that you want to travel.