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 the api provided by [full post rss feed](http://fullpostrssfeed.com/), which extracts 10 posts from the rss feed, it is basically hosted version of fiverfilter premium Api. Fivefilter regularly updates their templates to extract the content from the major themes and sites. 

If it is not able to extract the rss feed from any site, then you can add the template for that site, that means, template is something which will provide the details of tags and class to extract the content. For example, In wordpress, mostly the content will start from article tag or Div tag with some class. If you know any one programing language, then you can write your own rss parser and full content extracter using curl or any other method. It is very simple, you need some bandwidth to understand the concept.