In EHX v7.1 the Advanced Settings --> Panel Options screen GUI provides the option to add expansion panels to i-stations and v-series panels.  In v7.1 the ability to add / delete expansion panels is simplified by simply clicking on the "+" next to the Expansion panel option to add an expansion panel.  Clicking on the "X" next to an existing expansion panel will delete the expansion panel. 
The maximum number of expansion panels that can be added to a V-Series panel is 8.  They must be the same key type as the mains panel.
The maximum number of expansion or V-module expansion panels that can be added to an I-Station panel is 6.