Once you have added For ex. Google Map to its map repository (I think its not there by default anymore), you can use a software called MOBAC (formerly TrekBuddy Atlas Creator) to download maps to your PC. All the downloaded tiles need to be moved to MyDocs\.maps which is the common folder for map applications like ModRana, cloudGPS etc. Once the tiles for the area under view is placed in the directory Mappero would automatically pick them from the cached maps folder.

PS: Even MOBAC has disabled Google Maps and many others by default so try looking for the older version of the program with support for all the maps
Also I think keeping a cache of tiles from such repositories voids their rules of use, so be warned about that.