Some of the suggestions here are MUCH more difficult then they need to be. There is a fairly simple solution.
- Load the eBook in the Calibre editor
- From the File menu, select New file
- Type a file name and click OK
- Move the file to where you want it
- Copy/past your chapter
You can then edit this chapter as needed.
One thing you can do is save as HTML from the word processor and copy just the lines from that chapter and paste then into the mostly empty file in the ePub.