Hi Lucy,
As far as I know, there is currently no official, or unofficial publicly available solution to publish comments from Wordpress to Discourse.
So, it would require custom work. I am aware of two occurrences of private/unavailable WordPress plugins created for this purpose in the past.
Ideally (but depending on your needs), all the older blog posts could be synced with WP-Discourse, which can also be done with a custom Wordpress script.