Micro.blog really doesn’t have the concept of posts that only appear in the timeline, but there are ways to trick it into doing something similar. To add to what @jsonbecker said, you could have a special blog category called “Timeline”:
- Add posts you want to appear in the Micro.blog timeline in the “Timeline” category.
- Remove the default feeds under Account → Edit Sources, and add back the RSS feed for the timeline category.
- Customize your blog’s theme to exclude the timeline category posts from showing up on your blog’s home page.
Posting the timeline posts somewhere completely different (to Mastodon like @MitchWagner said, or to a separate blog) is another way to go.