Instagram confirmed that it is testing an option that lets you "repost" / "regram" public feed posts from other users to your Story.
For those who have access to the new feature, it will appear as "Add post to your story" when you tap the share icon underneath a post. If you do not want your public posts to be shared in this way, there is the option to change your privacy settings. Even if your account does not have the ability to currently reshare, you should still see the option now for "Allow resharing to stories" if you go to Settings >> Story Settings.
Zachary Shakkad, the app developer, pointed out on Twitter that when the post is added to your Story, you then have options for how you want it to appear. You will be able to resize it, rotate it, move it around and there are two options for how it visually looks: one that is a cleaner design with the original user's handle underneath, and one that appears more like a traditional Instagram post that you will see in your feed.
And also a better video of the feature pic.twitter.com/IQfFKMcCKf— Zachary Shakked (@zacharyshakked) February 7, 2018
This is one of the many updates Instagram has made to Stories as of late. And is one that has long been desired, as shown by how often people take screenshots on Instagram to add to their Stories. Keep an eye out for more updates to come.