Can we still add Windows OEM/System Builder licenses to an Enterprise Agreement after 90 days of purchase?
There is a general option to add OEM/System Builder licenses within 90 days from the date of purchase. However, for Enterprise Agreement (EA) customers, this option is restricted on so called “Additional Products”. As a general rule, everything that is not standardized within an EA can be referred to as an “Addional Product” (in many cases, for example, SQL Server, System Center, Visual Studio). Under EA, this option is not available for Enterprise Products (for example Windows, Office, CAL Suites), and therefore does not apply to Windows System Builder licenses.
Source: Microsoft Product Terms, August 2018, p. 78