If you wanted to do this on-premise/hosted, you could try the Managed Snippets Plugin; however, there are no ways to currently embed external assets via iFrames that is supported in cloud. Wish I had a better answer.
That being said, using the Apps Framework ... you can drop an embedded experience into a web page that a person can click on to launch a modal dialog with the iframe embedded inside it.
Hope some of that helps.
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