Add show more/read more to CKEditor Drupal
I don't think this is possible within CKEditor and even if it's possible you need to make sure that your theme understands the tags inserted by CKEditor which is not always the case.

This is because when the text content is displayed on the theme layer CKEditor is not involved at all at that point and Drupal, the Text Formats you define, views if configured, and the theme layer take care of displaying the text.

Possible solution: by default Drupal gives you the body text as "Long text and summary" with the widget "Text area with a summary" and hence your editors, by default, can click on "Edit Summary" on top of the editor to input summary text. I highly recommend you stick to inputting the summary text here and then the "read more" text in the body as most themes and views use that functionality by default. Some don't, but I see this as standard most of the places.

Good luck.