Is it possible to add an HTML Snippet to Numbers on OS X?
Not currently.

Numbers does not have a Safari or WebKit rendering engine to translate HTML, CSS and JavaScript so as to render content marked up for the Web.  This functionality is currently not supported in Microsoft Excel or even Google Sheets either.

See this similar answer related to Google Sheets on WebApps Stack Exchange.  The user there explains:


  Google docs won't actually render HTML, or execute any code pasted
  into it.