You can init Your map via mapViewer.config.initConfig. See
https://github.com/kubaszostak/mapviewer/blob/gh-pages/js-mapviewer/mapviewer-config.js

You can also add line somwere in your code:
mapViewer.mapServices.add("WMS", "http://wms.gdos.gov.pl/geoserver/wms?", "WMS Map Title");