Hi
if you have a virtual disk group it belongs to a virtual pool you can add another virtual disk group to a virtual pool
but best pracrice is to add another disk group with same settings. ( that is another 4 disks)
you can not converts it to a linear disk group.
I think you can create a new linear pool put the new disks in it (linear disk group) copy the data and the add the original disks
to the linear disk group ( can contain maximum of 16 disks)
Regards