What do you mean by “exporting comments under a specific post”?
Discourse has the concept of Topics and Topics are made up of Posts.
Are you asking about how to migrate Comments from one platform, to Posts in Discourse?
There are a whole load of guides on migration here: Topics tagged migration
You can pick the one for the specific platform you are trying to migrate to Discourse … guides exist for most of the main platforms.
If it’s about getting at all the Posts from a single Topic, you could try:
-
a Data Explorer Plugin query, or
-
use the raw format URL, e.g.: https://meta.discourse.org/raw/230008