Thanks Hugh!
I did try that but shortly after I realized that if you manually add an item/slide into the Menu (via the player) , yes it will show up in the menu, yes, it will get checkmarked when clicked, BUT because it was a slide that was manually added to be displayed in the Menu, Storyline doesn't know which slide it is "associated" with, so, when the user clicks on the manually added lightbox, it takes them to the 1st slide(restart the course).
I didn't know how to fix that so I ended up creating my own "lighbox effect" using regular slides, branching accordingly, and adjusting the size of the visuals to make it look like an actual lightbox. Because it's a "regular" slide, it does appear in the Menu and it does what its supposed to when clicked on from the menu.
In a nutshell, going forward, I'll just create my own "lightboxes"