Currently, BCMA does not support migration of SSH keys added to Bitbucket Server. Customers must manually add the SSH key for each user and with the number of users, this is not a very scalable task.

There is already an API endpoint for adding an SSH key to Bitbucket Cloud here - https://developer.atlassian.com/cloud/bitbucket/rest/api-group-ssh/#api-users-selected-user-ssh-keys-post