It is possible. The options can be found after clicking on the mock service in the bottom left part of the screen (unlike all other configuration options that are available in popups). There you can choose outgoing and incoming security configuration set from a dropdown.
It is hidden so well that I ran into the problem twice within three months, performed a research and found my own unanswered question in StackOverflow before a friend showed me the solution.
So I post the answer should anyone need that.