I working on OKAPI and I have to add page break feature in existing code.
It's not currently possible using Okapi alone. The Okapi filters go out of their way to avoid disturbing the structure of the document, and there's currently no functionality to add new markup during extraction or merging.
Your best bet is probably to combine your approach with something like Apache POI that is designed to modify the document. You can reformat it first using POI, then extract with Okapi.