Is there any way to tag a user to get his attention in some question?
My suggestion: do not target specific people simply because you think they can answer your question (people do that to me and, while I'm happy to help wherever I can, I also appreciate the ability to pick and choose which questions I focus on).
There are other ways to get attention for your questions, and really you should stop thinking of any single person as the best/only person who can answer it. (When your car is out of gas, do you care if the gas station coming up on the highway is Mobil, Shell, or BP?)
The most obvious way is to use a bounty, but there are many other ideas discussed in this long-standing meta question: