The Full Text Service settings under the Advanced tab when creating or editing a source allow you to manage how the full content of the imported posts is handled.

Some source sites that you will import items from may provide either partial feeds or full ones. The partial feeds typically provide an excerpt and then prompt you to click on the Read more link that leads you to the source site for the full content.
Using the premium Full Text Service could solve this problem allowing you to the full feeds for almost all your feed sources, whether or not they offer only partial feeds by default.
Use Full Text Service
Enable this checkbox to use the premium Full Text Service feature. This service creates a new feed by scraping the original site to bring in as much of the original content as possible.

Note: Use the Full Text Service with responsibility and at your own risk. This service will create a new feed by scraping the original site to bring in as much of the original content as possible using our full text script. Please remember that you should use this feature only if the source site permits you to re-publish the full content on your site with explicit acknowledgment from the original source. Always consult the source website’s terms and conditions for content sharing to remain respectful to the original creators.
How It Works
If you have just the Feed to Post add-on running, Aggregator will fetch only the items provided by the feed source. The content displayed as part of the excerpt of the converted post will be derived from the <description> tags. If the <description> tag is not available, the <content> tags will be taken into consideration.
However, many feed sources choose to provide only a limited amount of content, prompting users to click on a ‘Read More’ link to get the full content. With our Full-Text RSS add-on running, our code will visit the link provided in each item and try to ascertain the content you desire to display on your site. Since web pages across the internet are not built according to a set pattern of HTML tags, the Full-Text RSS add-on does not always succeed in guessing the content you wish to display.
Please remember, that you should use the Feed to Post add-on only if the source site permits you to re-publish the full content on your site with explicit acknowledgment of the original source.