Q: Is there any way to expand/grow a RAID array without having to backup and restore all my data?
Q: Is there any way to get the functionality of dRobo or XRaid2 or UnRAID in XigmaNAS?
A: Nice idea, very old story, few options as of 2012/12, but ZFS is a good one that is now becoming more popular.
If you have a ZFS mirror (RAID1), you can replace one disk at a time with bigger disks, thereby enlarging the array, sometimes without even turning off the system if you have the proper hardware.
The following alternatives create systems that allow the addition of more drives without having to backup and restore your data, the methods and technologies are either expensive or not user friendly for the most part.
If you have not done any of the above and have a simple RAID array you need to expand please read Q: What is the best, safe way to enlarge a SoftwareRAID array?