Copying a database that is in Retore status
You cannot use any backup statement to get a copy_only backup since the database is still restoring. At most the only option would be to use STANDBY mode but this would only provide you read access to all the data. 

You can get it to STANDBY by using the following command:

RESTORE DATABASE sample_db 
WITH STANDBY = 'C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQLSERVER\MSSQL\Backup\sample_db_standby.file';


This will put the database in a read only state and you could use SSIS to export the data. You can also use SELECT..INTO for the tables you want to another database. It is odd that SSMS is not going to let you use the Export Data wizard even though the data is accessible in a readable state (just tested trying to use that). 

Once you have gotten the data you want out you can put it back in NORECOVERY by issuing this statement:

RESTORE DATABASE sample_db WITH NORECOVERY;