How To Move Account Under Sub-Account

In Nintex AssureSign, how can I move an already existing Account underneath another Account? I only see a way to create new Sub-Accounts within an Account. I am not finding a way to change an existing Account to be a Sub-Account. Is this possible?

For example, say these 3 accounts currently exist at the same level:

  Account A

  Account B

  Account C

How do I move Account C to be a Sub-Account of Account A, so the structure would look like this instead:

  Account A

      Account C

  Account B