@bkgoodman This can be set from the fog web ui. FOG has a concept of storage nodes and master nodes. Only master nodes can collect images, you can deploy images from both master nodes and storage nodes. Where storage nodes are deploy only.
While the following deals with synology nas devices the same info can reference other NFS aware devices. Your target system needs both nfs AND ftp enabled to be able to function as a FOG storage node.
https://forums.fogproject.org/topic/9430/synology-nas-as-fog-storage-node