I would first want to question why you would want to do that. A sprint is a collection of stories that you ideally want to complete in a fixed period of time. It is not necessary that you will complete your sprint fully and that is exactly why we measure velocity.
If you had the option of adding all issues in sprints directly to fix versions then there would be a risk of adding an unfixed issue in the sprint to a release which is not ideal.
If you are sure that all issues in your sprint are fixed and ready for release what i suggest you is use JQL to search for the sprint and then do a bulk change to modify the fix version of all the issues returned from the search.
Hope this helps.
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