Do you mean load the iframe inside the div only when an element is clicked? If so you can delete the iframe src attribute from the iframe tag and set the src only when the element is clicked.
On the iframe:
<iframe id='ifr' frameborder="0" scrolling="no" width="550" height="400">
On the clickable element:
onClick='document.getElementById("ifr").src="add_dossier.php";'
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