I did migrate my system from plain ubuntu server to rockstor few weeks ago and finished a few days ago
My process was more than straight forward, if you read my posts you gonna understand.
My recommendation, if you have the disks available for data migration, create the pool and raid level you need from the beginning. This would save you a lot of time balancing your disks. Rockstor is not really good at the moment in giving you feedback how the balancing process status is. So you have to work with CLI anyhow if you wanna be updated.
If you do not have enough disks to create the pool with the right raid level to move your data from A to B the process I have chosen was.
I took a single disk 3TB which was empty and added it to rockstor. Moved data from another disk to it. Added the newly cleared disk (another 3TB) to the pool and changed to RAID0 because of disk space. Moved date from the third 3TB disk to the RAID0 pool and added the disk again. After adding all my disks and moving all my data to BTRFS on a rockstor pool I changed to RAID5
On my HP NL40 every balancing process of the data ( around 6TB) took between 12 and 24 hours, during this time it is better not to move or add new data to the system, otherwise it is going to take way longer. Worst case is, that your disk run full and balancing stops, happened to me as well, because I started the RAID5 migration with to less space available.