We have an active Microsoft Enterprise Agreement (EA). How can we add new PCs without operating system to the EA?

In a Microsoft Enterprise Agreement, Windows 10 is only available as an upgrade license. To add a Windows upgrade license to a device, this device nees a qualified base license installed. Thus, qualied base licenses need to be purchased for the new PCs, which can be FPP or OEM/System Builder licenses. New Windows 10 upgrade licenses can be reported with the next true-up, but can already be allocated to base licenses and uses before the next true-up

Source: Microsoft Product Terms, August 2018