
No you cannot. Use a form region.
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When you open an email by doing double click on it, I would like to know is the top panel of the email (header - indicate in the screenshot with a blue rectangle -) can be customized by adding text or other controls. Also in the context menu that appears when you click on the three dots button, I would like to know if I can add some options there. If that possible. Below I attach a screenshot and I indicate there the places where I would like to place these things (see blue arrows).
Also I would like to do the same in the preview area where the mail is shown when you select it from the messages list.
No you cannot. Use a form region.
The Outlook extensibility model doesn't provide anything for customizing these bits. There is no trivial way to get the job done. The best what you could do is to develop a form region (Replacement or Replace-All) which can substitute the whole inspector UI, so you could re-build it from scratch. Read more about Outlook form regions in the Walkthrough: Design an Outlook form region article.
Also you may consider putting a form region with your custom information at the top of the window, but right below this pane. See Advanced Outlook view and form regions from Add-in Express.