You would not be able to determine that with any certainty, especially on a per post level. Available RSS analytics and the analytics within the app via Google Analytics (based on how AppYet's "screens" are setup) are a different animal than web analytics. Not saying it isn't possible to aggregate them all to get a rough estimate as a whole to determine overall blog readership (but you may be double counting). In most use cases you want to have the separation (my website has x readers (and xx hits) and my app has y users (and yy screen views). In the end it all depends on what you want to track and for what reason and how willing you are to develop on your own to accomplish your goal.
One roundabout solution is to make your RSS feed truncated so readers via an AppYet app or an RSS reader need to click through to read the full story. This would enable you to view your web hits/analytics only to determine the traffic to the full blog post. Note that truncating your RSS feed comes with other considerations (for example, it can impact readership).