This could be a much longer answer but the short answer is:
1) Yes, you can add physical disks to a linear VDISK to expand the set size (see CLI manual for specifics). You cannot do this on virtual VDISK but, if you have enough free space in other virtual VDISKS then you could potentially delete the VDISK without effecting data availability. Then recreate it using larger set size and new RAID type as desired.
2) You cannot simply "convert" a VDISK from RAID5 to RAID6. For linear it involves moving the data to a new volume (host side copy), delete, and recreate. For virtual, the answer is the same if you don't have free space in the pool to absorb the deletion of the vdisk. If you do have free space in the virtual pool/vdisk then you can delete and recreate without effecting data availability.