Hey Boyan,
You are correct, the license will apply to the entire organization.
Once you add in the enterprise license for SM (for activating new features) in your organization it will automatically upgrade the entire organization from Legacy to Enterprise and the process is irreversible.
To answer your question, you cannot add paid licenses without converting. Legacy and Enterprise models will not co-exist in an organization.
You can create a new organization and apply the paid license to the new org. After that, move (re-enroll) devices that need to use the enterprise features to the new organization.