Under the place where you post questions, there is a textbox where you can enter email of someone who you think might know the answer to your question. I wonder if there is a way to mention a member of this site in that textbox to politely ask him/her to involve. I'm asking this since I cann't find some member's email address. If there is not such a thing, they could add it as an new feature.
Other than contacting the person in chat there is no way of "targeting" users in this way.
If your question is interesting and tagged correctly then, assuming that they are interested in those tags, then it's highly likely they'll see the question anyway, and if it's a good question they may well answer.
Questions are, by definition public, and open to anyone to answer, whether they have a registered account here or not. Also as the focus of the site is on questions and answers it's highly unlikely ever to be implemented.
If a user has edited a post you can "ping" them in the comments using the "@username" syntax, but using that to ask them to answer would be considered rude and the comment is highly likely to be deleted.