I might be a little confused about the nature of your question. You can edit your existing songs, and add a key designation either in the title or in the Key field (which allows you to sort using the key dropdown on the main library screen). To edit a song, tap and hold on a song, and select "Edit Song". If you are really asking about something else, please let me know, and I'll do my best to answer whether the feature is available or planned for a future update.
Thanks,
Mike
Thanks,
Mike