Is there a way to take an existing HTMl page and insert a preview of it into Libre Office Impress?
This depends on the size of the HTML page and how you want to display it. The easiest way is to create a screenshot and to insert the resulting JPEG / PNG file in your presentation. Because of the reduced size, fine details won't be recognizable for the audience, and of course you can't interact with the html page. So, there are two alternatives:
prepare images (screenshots) or magnified parts of screenshots and paste the into the presentation;
to demonstrate a "live" page, temporarily leave impress and the presentation demonstration and use the browser.
The decision depends on the main matter of the presentation: If it's solely the existence of a web site or its overall design, then it's alterative #1; if it's the functionality / usability / general usage of a web site / web application, it's alternative #2. Of course, you can combine both, too.