The RAID Level Migration is supported on specific platform configurations, you could try doing it in case you care using a third party board, but I cannot assure it is going to work properly.
Before doing this, backup all your data to avoid any troubles.
In the page 11 of the Intel(R) Rapid Storage Technology user guide you can find more information about this and what are the RAID configs you can migrate from/to.
The following RAID level migrations are supported:
• 2-drive RAID 0 to 3,4,5 or 6-drive RAID 5
• 3-drive RAID 0 to 4,5 or 6-drive RAID 5
• 4-drive RAID 0 to 5 or 6-drive RAID 5
• 2-drive RAID 1 to 3,4,5 or 6-drive RAID 5
• 4-drive RAID 10 to 4,5 or 6-drive RAID 5
Also, I would like to recommend you to check this article related:
Regarding the RAID expansion:
When you extend or shrink a volume, only a simple volume or a spanned volume can be modified: You cannot extend or shrink striped, mirrored, or RAID-5 volumes without deleting the volume and recreating it.
This information was found in the Overview of Disk Management check page 2 of this article: Overview of Disk Management
More useful information: Move a RAID 1 Volume to Larger Hard Drive for Boards and Kits