Increase disk space for iTunes on iMac

I have a mid-2010 iMac with El Capitan where my iTunes library is stored on two 3TB Firewire 800 drivss configured as a mirrored RAID set. The drives are close to full. What would be the best way of making the available space larger for my library? Can I concatenate disks to make the RAID set larger by adding to the existing drives or is it better just to fit a larger pair of disks?