What kind of RAID controller do you have?
I know that you can add disks and expand the RAID online with SMART controllers ('Course response time to any data on the array complety crawls while is re-striping the data
if you controller won't support this, I'd agree with garlane. Get a large external drive, attach it, and create a B2D folder on it. Do your backup, add the new drives, let the Array re-build, then see the Admin Guide under "Disaster Recovery" to do the restore
I am curious how you can be using the DLO with v9.1, since v9.1 doesn't support it?