Memory risers are the way to go with this system. Each memory riser card will allow you to add memory modules. It will be able to support up to 128 GB depending on how you have it configured. Otherwise, the systemboard wil be limited to 32GB.
Take a read of the following HP document.
http://h20564.www2.hp.com/hpsc/doc/public/display?docId=c01316946