It is indeed possible to OEM activate a non-OEM image.
How you do it depends on exactly what kind of OEM license it is, whether it’s OEM:SLP, OEM:NON-SLP or OEM:COA. Read this to see the differences.
‘Activation Backup & Restore 1.7 BETA 1’ can be used to extract OEM:SLP Licensing key and certificate which is tied to the manufacturer, the BIOS, the key, the cert and the OS from an original OEM image.
backup-cert.xrm-ms
backup-key.txt
You can then use the same program to restore the OEM:SLP key and cert to a non-OEM image (like GVLK aka KMS).
There are other tools and methods but you’ll have to do some careful digging.