Hi Gaspar
If you create the national courses in a Site level category such as the Miscellaneous category then users in tenants will be able to see and enrol in these courses providing Tenant isolation is turned off.
This also applies to Programs and Certifications
Be aware that if tenant isolation is disabled then users from different tenants have access to the same site-level courses, they could interact with one another (such as in forums). If this isn't wanted then using Separate Groups for each tenant Note you can use the audience sync to automatically assign audience members to a specific group
Regards