Only possible with t-sql commands.

When you create your backup you can specify a password for the backup file. Then when you want to restore the backup, the password must be specified otherwise the backup fails. One problem with using this feature to protect your backups is that the GUI, both Enterprise Manger and SQL Server Management Studio do not support this option. Also, when you create backups using maintenance plans this is not a supported option either. So the only way to create password protected backups is by using the T-SQL commands for both backup and restore functions.

http://www.mssqltips.com/tip.asp?tip=1108

MJ