Unlike in the native RE Import, you can import to the Do Not Contact (DNC) field in Importacular. This will work with either an existing email address/phone number or a new email address/phone number.
The screenshot below shows an import where all of the records in the import are defaulted to being coded "Do Not Contact":
You could also work with a Source Field if applicable or a Data Transformation.
To see a tutorial video that shows how this can be done with existing email addresses, click here.