No it can't. The only thing that a USB HDD attached to the Time Capsule can do, aside from being a wirelessly shared volume, is either back up the internal HDD of the TC or be used as a separate volume for Time Machine (in which case, the backup is built from scratch again).
Do as milk242 suggested if you want a RAID1 config with AirDisk/Time Machine. Although it's not officially supported by Apple to use Time Machine via AirDisk using an AEBS, it will still work and will yield the same results as a Time Capsule.