(Make sure your SAN storage is already presented to your server and Windows can see it)
Log in to Cluster Administrator.
Open up the resource group where SQL Server is located.
Right Click - > New -> Resource - Physical Disk
Add your new Disk.
Bring Cluster Offline (Right Click on resource node, click Take Offline)
Double click the SQL Server resource and click on Dependencies
Click Modify
Add your new physical disk
Save.
Bring Cluster Online (Right Click on resource node, click Bring Online)
You can now move database files onto the new disks.
Cheers.
-Alin