Quick and dirty way:
Edit the servicesDefs.py (located at ~/.config/blender/{versionNum}/scripts/addons/blenderGIS-225/core/basemaps
)
Add the following entry to the list of sources:
"STAMEN" : {
"name" : 'Stamen',
"description" : 'Stamen OSM',
"service": 'TMS',
"grid": 'WM',
"quadTree": False,
"layers" : {
"STAMEN" : {"urlKey" : '', "name" : 'Stamen', "description" : '', "format" : 'jpeg', "zmin" : 0, "zmax" : 19}
},
"urlTemplate": "http://tile.stamen.com/watercolor/{Z}/{X}/{Y}.jpg",
"referer": ""
},
Restart blender, voilà.
Todo: open an issue and eventually submit a pull request