Hi Michelle,
I did this for Adobe hosted SFTP so I'm assuming that it's do able for external SFTP.
You have to open a ticket with Adobe Support and ask them to set it up for you. You will need to provide them a ssh public key and IP that will be used to connect to the SFTP.
They need to authorize the campaign server with the SFTP server using a SSH key pair from the backend. Once this step is completed, you will be able to use the SFTP as an external account in workflow activities.
Thanks
David