There is no conversion from thick to thin or vice versa. You will need to recreate the volume as thin. Please check SANtricity System Manager for hep with shrinking volumes. You will need to use the OS's UNMAP feature to shrink volumes.
Yes, you can increase the volume size. If your volume group or pool is full you can add more drives to increase the size of the group or pool and then increase the volume size. This can all be done dynamically.