EDIT: I created a layer file (from the default ESRI Transportation Reference Style) that includes most of the signs included in the out-of-the-box sign inventory solution.
If anyone is interested in this layer file, please let me know. I would be willing to share it with others interested.
EDIT #2:
I've been adding custom symbols by either creating them with an Illustrator program or using the Snipping Tool. Saving the files as a .png you are able to add custom symbology for signs that out of the ordinary or specific to a certain location.
Use PNGs as ArcGIS Picture Marker Symbols | ArcWatch