Yes if you get 500 SP you get one automatically, but there is nothing blocking you!

I saw this article that has a script that can habilitate your slider - https://steemit.com/steemit/@alexpmorris/reminder-steemit-scripts-available-to-hide-show-resteems-enable-your-post-vote-slider-and-to-vote-past-payout

Do use your own judgement and research before installing any script ever for anything.