Customize the output of the Drupal 8 Aggregator module
I believe what you are looking for is the 'Manage Display Option" viewable from 

admin/config/services/aggregator/display

In other words from the aggregator admin ui.

Not sure how much control this gives you in reality. Also I am having issues with Aggregator appearing to strip out image tags from my feeds for some reason, so I am also considering using the feeds module instead that seems to have better support than the core module.

UPDATE 

Ended up using the migrate module which enables custom mapping of fields from a field to a content type of your choosing with custom fields etc.