How do you add a photo to an existing stack

I use stacks to organize similar shots for editing or for just minimizing the clutter in the organizer view. But, it is always the case that I find another photo that I would like to add to an existing stack. I cannot find any facility in Organizer to do this. 

I found a convoluted way to do this by first selecting a photo in the stack (any photo) and then selecting the additional photo. This creates a unusual looking stack; kind of a stack within a stack but it does include the photo. 

The must be a better way. Does anyone know how to do this. It seems like this would have been included in the stack dialog box but I don't see it.

Thanks.