You probably know Joomla! wrapper component. It enables to show in Joomla! whole "foreign" web pages or non Joomla! applications.
I like it.
You can do that with Jumi as well.
Just include into your php scipt that will be inluded by Jumi module (or Jumi plugin) into module position (or article) html iframe tag.
The general syntax of the html iframe html tag can be found elsewhere.
You can view not only external sources in Joomla! but you can also make non-Joomla! applications that you downloaded into your server running "under" Joomla. For example shopping carts, forums, etc. Just point src attribute of iframe tag to the index file of the application.
For "full screen" visibility it is better to use Joomla! wrapper. When you wish to show just a small "foreing" element show it via Jumi and iframe in a module position or in an article.