Can you link the data frame extent indicator box with your legend so that the legend displays a symbol and description representing the box?
I don't believe you can do this, but one workaround is to add a dummy polygon layer to your project which has no features in it, but has the same symbology as your extent indicator. Add that layer to your legend.
You cannot add a display indicator as an element to a legend in ArcMap.
You could build an element using graphics and just place it underneath your legend. It's usually not terribly hard to copy the formatting of the patch and text.