Add additional pictures to collage template

Using some of the built-in collage templates (as well as some from purchased packs), is there an easy way to add additional pictures that will automatically get input to the existing animations?

I thought I read a page on the website that said if you pick something that uses, say 5 pictures, then open 6 or 7 pictures, it would tack the extra 1 or 2 onto the slide, repeating the animation for them.  But as Im using it, all I get is an option to choose the pre-defined amount of pictures.

Ive also tried copying the final layer, pasting it and then changing the picture within it, but that just makes for a static layer, not integrated into the overall animation used for all of the other chosen pics.

Thanks in advance.

-Chris