-GN's skin just has a different name for defaults in his skin, he registers the element prefix as "blank" which is why he doesn't have the default numbers appear instead even though he has no "default-x" elements in his skin folder, he's also entered the file location for the defaults ("blanks"). This is done by typing down the following in the skin.ini file in the skins folder.
>hitcircle4 being the folder for the defaults
>blank being the default-x prefix
So e.g. I could do the same thing but instead give my defaults the prefix "circle" and osu would search for files listed under that prefix and use them as hitcircles, and to answer your question, all you simply do is treat default-x as a hitcircle. As seen in -GN's skin below.
Hope that helped!