Is there any available tools to migrate from sharepoint discussion board to discourse?
There does not appear to be a sharepoint importer in
It appears that Export sites, lists, or document libraries in SharePoint Server - SharePoint Server | Microsoft Docs might be of some help in figuring out how to export the data. What is most desirable is a full dump of the SQL database, not Excel files.
If you’re not a programmer and don’t have access to one, you can ask in #marketplace or see https://www.literatecomputing.com/discourse-migration/.