Question: How can this be done?
Now I use fulltextrssfeed.com and WP robot, but I don't really like it, because fulltextrssfeed.com posts only like 5 articles when it was more than 50 in the original RSS feed. And all the plugins I tested (FeedWordPress, RSS poster, WP-o-matic, WP robot) couldn't post full articles from every feed I used.

Answer: 
The feeds themselves don't contain the full text of the articles.
What `fulltextrssfeed.com` does is fetching the URL of the feed entry and extracting the full text from there. This is what you have to do, too - if you want all the articles
you can use <http://www.feedsapi.com/> or <http://fivefilters.org/content-only/>