Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello, good day and thanks for your question. I understand that you are looking into integrating a separate account into your Microsoft 365 Family.
For this, if you are the owner of the subscription, you can send an invite on that account and accept it so that it will gain benefits as part of the Microsoft 365 family.
If another member of the family is the owner of the subscription, you may ask the owner to send an invite for you to accept and be included in the family.
You can also see the Microsoft support links below.