This plugin uses popular GPL
Full-Text RSS library to pull full-text articles from the RSS feeds even if they don't provide it:
https://wordpress.org/plugins/cybersyn/
A notice: the CyberSyn plugin is a very simplified (pared down) version of my
CyberSEO plugin. So if you are planning to buy CyberSEO someday, you may want to play with CyberSyn first.