You have several options.
- remove the optical drive cage and install another hdd cage. So you can have 2x 8 disks in the server. you will need a sas expander card or another smart array controller for these new disks.
- get something like a D2700 disk shelf and a smart array card.
(I would not mix vendors in a single system environment)
server quicckspec for supported options:
http://www8.hp.com/h20195/v2/GetDocument.aspx?docname=c02171016
you may use a smartarray P410, P411, P812 or the 468406-B21 SAS Expander Card.