No, there is no way that I'm aware of to do that on this particular Message Board system.
I suspect you're thinking of a hierarchial system like that used in Yahoo and on very old Message Boards.
The phpBB3 set up is chronologically linear... posts will appear one after another sorted by the time and date they were made, irrespective of who they are addressed to.
In part it depends on what you're used to, I think. I know I find it very difficult to follow the flow of conversation in the Yahoo style model... and this "chronologically linear" method is certainly the standard method on the Internet at this point in time.
“I hope, Anuruddha, that you are all living in concord, with mutual appreciation, without disputing, blending like milk and water, viewing each other with kindly eyes.” (MN 31)
What is the final conviction that comes when radical attention is razor-edge sharp? That the object of the mind is mind-made (manomaya). (Ven. Ñāṇananda)
"If you have always believed that everyone should play by the same rules and be judged by the same standards, that would have gotten you labeled a radical 50 years ago, a liberal 25 years ago and a racist today." (Thomas Sowell)