You can add Custom or Standard fields of any object that you have selected to your email template. You can add only those fields for which you have access to in Salesforce.

You can do this on the Setup Email Content screen (third sub-tab of the Send Campaign wizard). To add the fields to the email template, drag and drop Text, Text section or Quote tile to the template. Then double click the tile you just added to the template, and click the Merge fields button. You see all the fields listed in the merge fields drop-down that you have access to. Click on the field to select it from the list and add it to the Email Template.