it depends on *WHICH* isilon, there are huge differences in the generations....the isilon specs are a HUGE PITA to find.
isilon seems to favor direct attach but physically proprietary backplane/expanders; they are standard SAS and should work fine as is.
the biggest problem with them is if you want modern parts; because the chassis and expanders are often proprietary as f*$^, putting standard parts in is a b%($. they often have supermicro boards but with EMC bios.