We have already discussed about showing Jetpack sharing buttons inside your posts, pages etc by using a simple shortcode. Now there’s a new plugin in WordPress Plugins Repository by developers of Jetpack Sharing Shortcode Plugin.
So for extending your Jetpack and making it to show a subscriptions for via shortcode, all you need to do is install and activate Jetpack Subscriptions Shortcode plugin and start using [jpsub] shortcode inside your posts, pages, custom post types etc.