Yep, you can use preheaders in your HTML for this. This would be just below the head and just above the body, and would look something like this:
<body style="background:#f1f1f1; margin:0; padding:0; display:block !important; width:100% !important; " ><div class="mktEditable" id="Preheader" align="left" style="color:#f1f1f1; font-size:1px; line-height:0; font-family:Arial, Helvetica, sans-serif; " ><font color="#F1F1F1">
YOUR PREVIEW TEXT.</font></div>
One thing you'll have to worry about is how to make the preheader trump the "view as web page" html that Marketo inserts. If you are using this for emails where you have that option, then you'll likely need to recreate the "view as web page" functionality with some HTML. Here's a good relevant thread on how to do that:
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PN6NAAW