I know this is little old, but I just registered to the site
When you say ‘currently their san is set RAID5’ I believe you are talking about Network-RAID5.
You can add second node to the same cluster, however it will take a lot of time to restripe the data between the nodes. After the restripe is complete, you can then change the NWRAID5 to NWRAID10.
Or, you can create a separate cluster in the same Management group and move few volumes to the new one.
With NWRAID10 you get less than half the total storage, yes around 1.5 TB per node.
Hope this helps, please let me know if you have any questions.