What you’re asking a too broad to decently fit in WPSE’s Q&A format, but I’ll try to sketch an approach that could work.
From what I understand you have 10.000 database items that are not WordPress (custom) posts. You still want to present them as WordPress pages and enable comments per item. I’m guessing that your database is not static, so a one-off action to read them into WP is a no go.
So the solution would be to generate a WP post at the moment the database item is first called upon. Sketchily:
- Register a custom post type that holds only a unique identifier of your database item (and allows comments).
- When a database item is called upon, check its identifier. If there already is a post with this identifier, that’s fine, else create one with
wp_insert_post - Build a special template for this custom post type that draws more data from your database using the unique idenfitifier and retrieves comments from the WP database.
Beware that whatever your approach is, you will be creating dependencies between two databases, which will make future maintenance more difficult.