If you're not using SQL Server 2000, you should be avoiding aliasing users. sp_addalias is deprecated functionality. I don't even think it's available in SQL Server 2008 or higher. Also, the sysusers table is just there for backwards compatibility. You should be referencing sys.database_principals if you're on 2005 or higher SQL Server systems. You'll note that there's no alias functionality there.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood" - Theodore Roosevelt