Add your own layer by importing a file such as KML
See National Library of Scotland (NLS) website as an example
Glenn Gengel commented
I am creating an overlay of two very old maps that no longer have references in the base map of Georeferencer. Using old USGS topo maps (1953), I have found and marked the features that I need to get these images placed correctly. I have all the KMLs marked on Google Earth and need to a GeoTIFF to complete my map. Then I will have aligned the old mercury mine shafts of New Almaden, to 150 year old surface map to current topography. Without KML import or choosing a different georeferenced base map, this project dies.