Hi @Anthony BOUQUET · Welcome to QnA platform and thank you for reaching out.
If you don't have access to source directory, you can't migrate the subscription to an owned target directory. To transfer a subscription from one directory to another directory, you need to have access to both the directories.
I would suggest you to open a support ticket to get Microsoft Data Protection team engaged for this issue. After providing required information and evidence to prove the company ownership on the subscription, they can help you with migrating the subscription to the company owned directory.
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