We have already responded to the question in your support thread. For your convenience I'm posting the replay here as well.
Indeed the RadEditor is converting the & character to its HTML presentation, but this should not affect the links that you add, because when the HTML in the editor is rendered the HTML presentation will be replaced by the actual character. Please review the attached screen cast. I have tested this behavior by adding query strings to my links, which were in turn properly rendered in the public part of the website.
In order to disable the converting special character to HTML entities you must disable the respective content filter (ConvertCharactersToEntities) of the editor: Editor / Built-in Content Filters
. To be able to modify the editor within Sitefinity you need to take the approach described in this blog post: Customizing the Sitefinity WYSIWYG Editor
Also there is a related forum thread that you might need to look at: Hyperlink Manager converts query string &
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