Adding a domain account onto a workgroup server will not work.
Rather create a user account on those workgroup servers, and give them the necessary admin permissions.
Once done, on your BEWS server, create those accounts in BEWS under Network tab as Server1\accountname (password), and Server2\accountname (password).
This will allow you to use that username/password when you back it up. Try browsing to those 2 servers in BEWS, and use the username/password you just added in.
Obviously have the SQL agent installed too.