Is there a way to add an email to an existing case easily from within Outlook? I am using MS Outlook 2010
Salesforce for Outlook won't do this - you can only create a new case. I'm pretty sure you'll need to go for a paid integration to get this functionality. There are a few Outlook integrations on the AppExchange - LinkPoint is one that I have heard consistently good reviews of, though I've not ever implemented it myself.
I believe Salesforce for Outlook will meet your needs. It allows you to attach emails to many different objects, including cases. I just tested with MS Outlook 2010 and it's working great!
A few wrinkles: It looks at the email address in the From field, and if it doesn't find a contact or lead in your Salesforce org, it won't display anything (In that case, you'll have to do a search instead). If it does find a match, it will display related records (including cases), and you can click the + email icon to add the email to the case.
It adds the email as a task, not within the Emails related list.
Hope this helps!