Is it safe to add garlic powder to sesame oil?

The concern with garlic in oil leading to botulism is about long-term storage, usually in the context of garlic oil as a 'shelf-stable' condiment; the botulism needs time to grow in the anaerobic environment provided by the oil. If you're making a marinade and using it within a few hours or a day or two, as marinades tend to be, and especially if you're keeping it in the fridge, as marinades tend to be, you should be fine.