Please see below the answers to questions.
We are planning to add an expansion shelf to one of the existing DR6300 and had a question whether or not the DR system be available when it is adding an enclosure.
<Yes the DR should be available The DR won’t go offline or the filesystem in read only when adding an enclosure>.
Also, does the system try to rebalance when it sees the newly attached storage? Will this impact normal operation of the DR system?
<A rebalance will not automatically occur. space will just become usable after the process. However, it may prefer to use the new space over the old space until it is balanced>