There’s not currently a way to do that. Micro.blog can show replies to your post from Mastodon, but it can’t show replies to the copy of your post that is sent with cross-posting. When you see Mastodon replies on a blog post (such as one of my recent posts) it’s because people are following the ActivityPub username, not following a separate Mastodon account.
We might be able to add support for showing replies from a different account, but it would take some thought because you’d probably want to merge in replies from Micro.blog too.