Hi, I'm the author of the clock plugin.
The way it's done in the plugin is a custom animation, selection bar and menu code, not using Rockbox's menu code. The only way to get it into the Rockbox menus is to add support for it in the menu code (something I wouldn't mind trying in the near future).
So if you aren't a coder you can't really do anything to get that in Rockbox itself except wait, sorry.