1. Yes, you can expand existing disk pools with the new disks.
2. Yes, 'Expand' to add more capacity to the virtual disk. (Click on Virtual Disks > Select the Virtual Disk > Click on Expand.)
3. Yes, you can migrate the LUN with Navisphere Express. (Click on Virtual Disks > Select the Virtual Disk > Click on Migrate)
However, please note you can not select the Destination Virtual Disk in Navisphere Express it will automatically select it and take over the Information of the Virtual Disk. (Virtual Disk ID, Host Info etc.)