Cris,
Yes, you can add another drive and not have it in the storage pool.
When you attach the drive and start WHS, it will ask you if you want to add it to the pool or not; if you don't want it in the pool, you can then, (after saying 'No',) go into the disk management on the server and apply a drive letter to the new drive.
There are advantages to having a disk outside the pool, it is immune from the current corruption bug, it is available if you want to do a manual backup of your backups, or even just as fast storage.
Colin