You need the complete manual on the RAID1 system you plan to use. Is it built into the mobo's chipset and BIOS? Or, are you using an add-on PCI card to be the RAID controller for hard drives?
On current RAID systems built into mobo chipsets and BIOS's it is common that a RAID1 system can be created by simply adding a second HDD unit that matches the first, and then having the RAID management system copy all the data over to create the mirrored HDD, then alter a few key data items on each disk so that the pair become a fully functioning RAID1 array. In fact, this process is the basis for the method used to repair a RAID1 array with one faulty HDD that has to be replaced. But you really should read the manual for YOUR RAID system to verify exactly how this can work for your system.