Turkish Airlines offers me on their website to convert my bonus Miles into status Miles. What miles are applied to get a ticket as a reward?
Status miles count towards earning status, and also earn you redeemable miles. Bonus miles do not count towards status, they are only redeemable. So say you fly 1000 miles: there might be a "double miles" offer but typically the second 1000 miles is just bonus miles, not status miles. Miles from credit cards or non-flying partners (eg rental cars) are usually only bonus miles.
If you are not going to be close to a status level, you should not take this deal. I actually find the Turkish miles levels to be confusing but I think you need 17,000 miles to make Classic Plus. If you're at 16,000 and don't plan to fly more, and you think the benefits of Classic Plus (use the business class checkin lines and business lounges on domestic lights, for example) are worthwhile, then giving up some of the redeemable miles (eg exchanging 5000 bonus miles for 1000 status miles) would be worth it to you. 1 to 1 would be even better but I doubt they're offering that. However if you're at 5,000, chances are it won't be worth it.