This article discusses how to create custom fields in Campaign Monitor and how they can be applied to email templates.
When going through the process of creating or updating an email template with Campaign Monitor, text components in the email template have an option to "Insert" which includes "Custom fields". Here you can access custom fields that have been defined on the subscriber list and bind them to the email template.
To create Custom Fields so that they're available when creating an email template, go to a Subscriber List record in Salesforce and select "Field Mappings". Here you will see all connected custom fields that have been created in Campaign Monitor to reflect their Salesforce values. You can also create new Custom Fields, by clicking "Edit".
Then click on "New Campaign Monitor Field".
Select the Salesforce Field that you'd like to map to Campaign Monitor.
When the new field mapping has been created, you can click "Retrospective Sync" to map existing field values to Campaign Monitor.