Since no one else had responded yet, I will. Please excuse me if you already know this info, I'm not sure what led to your dilemma, but this is what I'd do.
If adding a disk shelf to the DD890 is not an option, then I believe the quickest and cheapest way to get more capacity is to use the local disk storage, as you suggested. You don't mention how much storage you have available to attach to the media server, though; it may not be advantageous to do this, so I would suggest you look at longer term alternatives. For instance, if you are getting a good average deduplication rate, say 10x, and you only have 5TB available on the alternate storage, then you would not save yourself much space on the DD.
There is no need to do a restore to get the data off the DD890.
Create your disk storage unit on your media server first. Be sure the media server has access to the DD890, too.
Then just go into the catalog on the Java GUI and select parameters (disk type, start/end dates, etc.) that would give you a list of backup images appropriate for being moved. I'd suggest looking at any file-server backups that may contain media files (images, music, video); they don't dedupe well and would just be eating up your DD890 storage. Change the "Action" at the top of the window from "Verify" to "Duplicate", then highlight the images you want to move. Right-click, select "Duplicate", and be sure to click the box next to "Copy 1", so that this new one becomes your primary copy. This is how you will be able to go back later and delete the images from the DD by sorting on the "Primary?" column. Select your new disk storage unit and leave the rest of the options at defaults. Click on OK, and you should see a duplication job running. The DD890 will "rehydrate" the files as they are read from the disk, so the DD and your media server CPUs will be quite busy moving data across to the new disk storage.
Once the duplication finishes, go back to the Catalog display, select the same parameters (especially the Disk type, start and end dates), and sort on the "Primary" column. Highlight the ones that are no longer Primary, right-click, and select "Expire". Depending on how many images, it may take 30 minutes or more to expire a few thousand images. You will see an "Image Cleanup" job run in the Activity Monitor just after it has actually finished updating the database and just before it refreshes the Catalog display.
After this is complete, you will need to run a System Clean on the DD890 to actually remove the expired data from the dedupe pool. Again, if you are deleting a lot of data, this could take a really long time and the DD's CPU will likely be very busy, which may impact your other backup processing.
Longer term options for backup image storage would be to work with EMC on the Data Domain Extended Retention option, which will let you add more disk shelves than normal and will shut them down once they fill up. Another option would be to use the DD890 for more temporary, short term storage and to use a tape library and multiple tape drives to offload the data as it ages.
