EricSnyder526, To answer your questions, yes you can do while the server is up and running, as well as adding them all at once.  As far as timing goes, there are multiple variables, such as traffic, load, controller, as well as data, but it can take a while to complete. The page here  will discuss the process.
I have a PowerEdge 710 with eight 2Tb disks in a Raid6 array with one virtual volume. I want to add and additional 6 drives (same make and model) to the array to expand the Raid6 and the virtual volume. I AM running OMSA. Anything to watch out for?
Expanding Raid6