Its 99.9% likely that the DAS supports capacity expansion whether online or offline. Online means it will expand the capacity onto the new disk(s) and still allow access to the data while it is rewriting/reorganizing the data across all the drives. Offline means it will not allow access to the data until it completes the expansion. This is something to check if you need constant access to the data.
1) What do you need the DAS for?
2) Is the DAS software Raid or hardware Raid and how does it connect to the PC(eSata, FW, ethernet)?