To keep my parts organized, I'll start a new library. If you have your own parts library, open that one up instead. The reason for doing this rather than editing the default libraries is that if you update Eagle, you wont forget that you have to go back to the old library and copy it over. Better to just keep all custom stuff separated!
Save it somewhere in your Documents folder or wherever you keep your files...
OK now keep this new library open but go back to the main Eagle window (not schematic/board/library) and browse in the folder hierarchy until you find where you found that close-but-not-quite device. In this case, it was in 40xx
Then find the device that you want and copy it to the library (the new one, you just made!)
OK now you have a copy of the 4093 device in your own library - again it has SOIC and DIP packages already