An iframe should be used to load a URL (a fully-formed web page) rather than just a video (although a video could certainly be the only element on a web page).
However, Juicebox does not support iframes or videos so it would not be possible to insert an iframe or a video into a Juicebox gallery in-between images. (The 'config.xml' file contains the configuration options for the gallery and the image data but it does not contain regular HTML code.)
Please see this FAQ: Will Juicebox load things other than images?
If you want to add an iframe to a Showkase page, you can do so by adding the iframe code in body of a page via the text editor. Just click 'Source' on the editor's toolbar to enter source mode (to tell the editor that you are going to enter HTML code rather than just plain text) and you can then enter your iframe code such as the following:
<iframe src="http://www.example.com/index.html" width="800" height="600" frameborder="0" scrolling="no"></iframe>