Use edit, split at markers.
Page breaks can be inserted as css or style commands. You can check out http://www.w3schools.com/cssref/pr_print_pageba.asp
, for one. They don't always work as expected (like what else is new?) And if the page is resized, you end up with a break where you may not want it.
If you gotta have page breaks and are into formatting, you are probably more into PDFs, where everything is fixed, rather than epubs where darn little is fixed, and is designed to reflow based on resizing the page when the user presses the S, M, L button.
From what I understand, not being a Kindle guy, not all of the epub stuff translates especially for the earlier Kindles.