Can I clone an ESD.dmg file to an external disk without having admin password?
You can just set a new admin password and then you're able to enter the password for cloning the ESD.dmg file (which you'll then perhaps not even need at all).

You can set a new password by doing the following:

1) Power on the Mac while holding down the Cmd key and the S key

2) When the Mac has booted into single-user mode, type the following commands:

mount -uw /
launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist
passwd roger


You need to replace "roger" with the actual account name for your admin user.

When you're done, restart the system by typing this command:

reboot


Now you should be able to login with the newly entered password.

For others: If you're running a newer version of macOS (10.8+) you can do the same in a much simpler way by holding down the Command and R keys while booting up to start in Recovery mode. From there open Terminal from the Utilities menu. Enter the command resetpassword and you'll be guided through the rest of the process.