Hi there -- Instagram is already a customization option in our social follow block!
You'll find those options by dragging the Follow block into your mailing from the drag & drop content block options along the left side of the editor. You can complete the Instagram url with your profile information to make it easy for subscribers to hop over to your Instagram profile.
If you're referring to the share icons in the footer of your emails, those make it easy for your subscribers to share your email to their personal social profiles or via email. We don't currently have instagram as an option there.