You can post any of your Steem posts you desire. You just find the "niche" which is basically the same as "tags" here. There's an area to put the link to your steemit post, so that it shows the content has already been posted somewhere else. But there is no penalty or downside to that, you're just being open about it not being original content to narrative, but it's still YOUR original content. I'd love to see some of you work in Flash Fiction - up to 1000 words and you're good to go!