I think to get get_field and get_sub_field functions in functions.php is impossible now. But you may use get_field_objects($post_id) function to use in functions.php. Just print returned array and use (parse it) as you like!
By the way you can use get_post_meta and get_user_meta functions against get_field, but I couldn’t use them to get sub_field of repeater in functions.php file.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users