The short answer is that you can't. Learn more below!
Bot Subscribers are users that have messaged the page or have opted-in through one of the growth tools that are attached.
As soon as somebody messages your page, you can message them back, because they're your Bot Subscriber. If you want to convert them into Bot Subscribers, all you have to do is give your Page Subscribers a reason to message you (i.e. offer them something they can't already get by just being a Page Subscriber), then let them know about your offer. You can use our growth tools like the CGT and widgets to expand your audience within Messenger.
Learn more about ways to gain more subscribers on our blog!
Hope that helps!