Convert Login from Windows Authentication to SQL Server authentication?
I think your best bet would be to script out the server level permissions granted to the Windows Login and save the SID, delete the Windows Login and recreate it as a SQL Login with the same SID, and finally grant the permissions back on the Login. 

This way the login will have it's server level permissions recreated and retains it's database level permissions since the login will match the database user via SID. 

    CREATE LOGIN [LoginName] WITH PASSWORD=N'PASSWORD'
            ,DEFAULT_DATABASE=[master]
            ,CHECK_EXPIRATION=OFF
            ,CHECK_POLICY=OFF
            ,SID=[SID]