Use sp_addexternlogin to assign an alternate login name and password to be used when communicating with a remote server.
sp_addexternlogin remote_server, login_name, remote_name [, remote_password]
remote_server is the name of the remote server. The remote_server must be known to the local server by an entry in the master.dbo.sysservers table.
login_name is an account known to the local server. login_name must be represented by an entry in the master.dbo.syslogins table. The “sa” account, the “sso” account, and the login_name account are the only users authorized to modify remote access for a given local user.
remote_name is an account known to the remote_server and must be a valid account on the node where the remote_server runs. This is the account used for logging in to the remote_server.
remote_password is the password for remote_name.