Proboards does not provide a way to export data. This is done intentionally to prevent you from leaving their platform.
While technically possible, it’s a lot of work to retrieve all posts. You have to scrape every user, topic, post and like (which is made challenging by Cloudflare). You then have to sanitize the data by fixing weird HTML, quotes, URLs, etc. Finally you need a custom import script to move the scraped data to Discourse.
You basically need to decide if the effort necessary is worth it unfortunately because there is no easy option