Hello @Grácián Gy ,
Thank you for your question. I understand you wish to associate a subscription to an Enterprise agreement enrollment. In order to add an account of different domain to enrollment the authentication level set on the enrollment must be Work or School Account Cross Tenant.
An enterprise admin (read and write access) can update the authentication level. Once it is updated you can add the account under the enrollment. And this will automatically associate the subscription under the account to EA. Please refer mosp-ea-transfer document for detailed explanation.
In case of further queries please raise a support ticket and the tam would be able to assist you further.
Kind Regards,
Divya.