I guess Facebook heard you because just 1 month after you posted it, it now happens by itself when you click "share" (unless you uncheck the option). Well, as long as you abide by the following rules:
- The secondary post must be completely public.
- The original item must be an external link and not a Facebook post.
- You share the secondary post on a wall (yours or anyone else's) and not in a private message.
If you stray away from any of these rules, the original answer still applies.
Officially you can't. Facebook treats it like a comment of sorts, so it's just like you can't officially share comments.
However, you can "cheat" Facebook by sharing the literal link i.e. instead of the
share button, get the URL from the post's timestamp:
Hey everyone, see what my friend wrote:
It doesn't offer a preview, but it works...