I dont think that is not possible in TradingQNA,
If i am not wrong, Quora started as paid answering service rewarding people to answer more questions and providing ways for it.
Honestly i would let the crowd sourcing model of 'interested people' will choose the question to answer option and also the option to vote for good questions, better than inviting people
You have the option in TradingQNA called a Private Message,
Once you post a question, take that link and send it to whoever in your favorites (or any one for the matter of fact) and request them to answer.
All you have to do is search their name in user list and choose the message symbol like a mail icon. You will be taken to the Private Message window. Paste the link there and send a request.
Sometimes, the user list will not give you the search result for some names, example your name "N’ cannot be searched.
In such cases, directly type in the address bar as http://tradingqna.com/message/N
You will get the private message window.
P:S, Many send me Private Messages regarding their doubts and sometimes asking me to answer some questions 
lol
This is not possible at the moment, but is something we can definitely add soon.
sometime, one may miss a question. Also, probability of answering the question increases when one is specifically asked (behavioural change as per research papers).