At the moment, it is not possible to transfer from one account to another. The reason is that when you connect an inbox to a particular user account, you are granting many permissions during the OAuth process.
Besides simply granting access to the existing account that the inbox is connected to via our Link Accounts/Single Sign On Feature (or perhaps changing the email associated to logging in with the account), the only way to actually move it from one user account to another would be to remove/delete it from the existing account, and then re-adding it under the user account you want the inbox to be managed under.
If you have a collection of inboxes under a user account, and perhaps due to an employee leaving or other reason you need them under another account, you may consider simply changing the login email for that particular user.