If the IG account you're trying to add is owned by another BM, you'll get an error which mentions that the account is already owned. If this is the case, there are two options that can be done to gain access to the IG account.
The first thing that can be done is to ask for partnership to the BM that owns the IG account. In order to do that, a person with admin access on the BM that owns the account should click on the "Assign Partner" button after finding the IG account in their own BM.
The second thing to do is to get in touch with the BM admins of the BM that owns the IG account (if the owner is known), asking them to remove the account and to add it to your own BM by following the steps mentioned above.