You can add a feature:
In the section: Email account settings - Filters I can create filters. It's OK.

But I can only paste the filter it contains: From, To, Subject. I must to insert header contains :contains
if header :contains "X-Spam-Flag" "YES" {
fileinto "Spam";
stop;
}

I am restricted only: From, To, Subject

How do I do it? Can you add it to the new version?