What you want is quite possible in ROS but you might not be able to do it in a live environment unless you have a P4 computer that support hot swapping of hard drives.
Go ahead and add an IDE Hard drive to your system. Make sure the HDD is not configured as primary drive, that privilege is reserved for what ever drive housing the ROS system files.
Now log into your router with Winbox, goto “/store disk” and format the new HDD. Then goto /store and copy the existing userman from the primary drive to the secondary drive and activate the secondary drive version.