You can now repost a story you were tagged, directly through Postgrain!
This feature is available inside Direct Messages tab and all you have to do is click Add this to your story and then Confirm:
You will be redirected to the New Story tab, and so will be possible to edit repost any way you want: with stickers, text and even drawing with the pen, just like on Instagram.
*It's important to remember that Postgrain can't repost a videos on Instagram Stories yet. If it's useful, you can still repost the first frame of the video.