Looks like these widgets have been discontinued. You can vote for this feature request here.
I found a tutorial in which the author has explained creating a Show More option using the Accordion panel. This might not be the solution you are looking for but it can probably provide you with clues to consider options within the current limitations of Adobe Muse.
Thats a good idea visually. But will it work as a regular "show more" button would? As in not load everything unless "show more" is selected?
I'm thinking is the perspective of loading this page faster since there is a lot on content to load.