If you need to create modal/popup window in WordPress admin, its easy. You can use buit-in thickbox jquery library.
Load thickbox js and css by calling function
<?php add_thickbox(); ?>
Add a link to html with class name ‘thickbox’ example below –
<a href="#TB_inline?&width=300&height=300&inlineId=my-content-id" class="thickbox">Open Popup</a>
Then add popup html,
<div id="my-content-id" style="display:none;"> <p> This is hidden content! It will appear in Popup when the link is clicked. </p> </div>
Now when you click on a link to open popup, above html will display in popup with overlay at back, you can customise as per requirement, check codex for details.