You can edit the Symbol Legend shapes for any of the Plan Shape stencils in Visio by following these instructions.
By design the symbol legend has been converted from Visio shapes to file type WMF vectors and text. This was done so the legend could be easily resized and work on any page scale. You can’t do that with grouped Visio shapes and text. This makes for a nice, clean, lightweight shape but it also means that you cannot edit this shape.
Note: Notice that there are two Symbol Legend shapes in the stencil, "Symbol Legend" and "Symbol Legend Editable". This first method will focus on the "Symbol Legend Editable" shape.
The Stencil will open in the Shapes window:
Note: DO NOT Ungroup the shape. Use the "Open Group" command as instructed.
Note: If you want to add a new shape/symbol to the Symbol Legend Editable shape you can drag them over from any other D-Tools stencil. To do this you may need to click the View tab and then the [Arrange All] button:
If you do add any existing SI 2017 shapes to your custom legend (vs. adding/drawing your own shapes) you will need to remove the coding from the shape (the coding that makes it smart and linked ot an SI 2017 Product). The easiest way to do this is to select the added shape and then from the Developers tab click Operations->Combine:
Now you will want to update the Symbol Legend shape with a .wmf (Windows Metafile) of the Symbol Legend Editable shape you just modified.