The only way you would destroy your existing drive pool is if you used the "Detach" button on the storage screen, and then checked the box for "mark disks as new". I don't think you're likely to do that. What do you want to do? Do you want the two new disks to be part of your existing pool, or do you want to create a new pool with them? Either is possible, though the former would end up with a kind of wonky pool configuration. In both cases, you'd go to Storage -> Volume Manager. If you want to add the new disks to your existing volume, select it under the "pool to extend" dropdown, otherwise leave it blank. Add your disks (click the + button), drag the volume slider to two rows x 1 column, and make sure the dropdown on the left says "mirror".