There is not a built-in way to modify the email preview header text.  However, with HTML knowledge, there are ways to work around this.  Since preview text is pulled from the first few lines of text found in the email stationery, you can add HTML code to the body of your email stationery or within an email message itself (assuming you do not want to have to create a new stationery for each email) to pull in the desired preview text.  It is recommended that you select a blank email stationery and that you place your stationery code into the HTML body of your email message instead.  For your preview text, you would need to the following code after the <body> tag in the HTML screen.  For example:
<body>
  <div>
    This is the text I’d like displayed for the preview text.
  </div>
</body>
Please note that the information listed above is not a supported function.  For tips and guidance on working with email pre-header text, check out the Luminate Online Community.