I was able to stack as many cap blocks as I wanted, simply by following the steps below:
Start with any script with two or more blocks.
Make a script with a single block and whatever cap block you want to insert.
Drag your cap block script into the middle of your initial script.
I seem to be unable to add photos to a post or reply, so I cannot show what it looks like. I hope my description is sufficient. (I wonder whether this limitation is because I am new to Snap!)