You can use the functionality for category export and import. Since it’s a command line tool you will be unable to use it from your Discourse for Teams instance. But this will work:
- Create and download a backup from your Discourse for Teams instance
- Restore it into a self-hosted temporary forum
- Move all categories you want to keep into a single category
- Create a category export file using the command line
- Import it as a category into the destination forum
- Delete the temporary forum