Hello hollosch !

If you have the URL for it then you may add the item to the Social Sharing menu in the Studio->Navigation->System area. The link for Twitter looks like https://twitter.com/share?url={url_encoded} so you need to transfer the {url_encoded} element there.

PS As I know Instagram has not so simple "share link" system (it is available for popular accounts or for the payment). But maybe it obsolete info.