Hi,
if you import the subscribers and can set that value in a custom field (see Subscription/Custom fields), you can then add that content on a newsletter using the {profile_x} tag, where “x” will be the number of the custom field you want to insert.
If you need to generated on the fly a custom content per subscriber, you need to use a filter to change the message. In this case, you can define whatever placeholder you want and then replace it. See more about those filters here:
https://www.thenewsletterplugin.com/documentation/developers/dev-newsletter-hooks/#filter-newsletter-message
Does that help?
Stefano.