Readonly database restoration failed

In one of my project I was working on SQL Server 2000 to SQL Server 2008 upgradation. It’s a side by side upgrade so I need to restore the database from SQL Server 2000 to SQL Server 2008, in which some of the databases are readonly. I know I will be able to restore readonly database however in this case when I tried to restore the database in SQL Server 2008 I got the error below