First, start by creating a custom prospect field. You can do so by accessing your initials then clicking "Settings and Billing".

Form here, select "Company Settings" then "Prospect Fields" and click the "+ New Field" button. In the example below, I've named the field  {{Alternative_email}}:
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Second, in the main prospects tab, add the e-mail under the preferred column as visible below:
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You can also update a CSV file with the data column as shown below. Please keep in mind that that the titles should not be separated by commas but rather in different columns:
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Finally, access your campaign, open the template un-link it, and click on the CC and insert the {{alternative_email}} variable there. It will parse the information from your prospects' tab the same way as {{first_name} etc as shown below:
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You are all set.