Yes, you can include line numbers by using custom documents. We’ve loaded an example template to our website here for your convenience, but if you already have your own custom document, you should be able to add the <<LineNum>> tag yourself instead of modifying our example template.
See the screenshot below for where to find the <<LineNum>> tag. Note that you must have already linked your custom document to a data source otherwise the “mailings” button will be greyed out for you and you won’t be able to select the tag.