Generally, the blank is epoxied to the tube, but it can be done. I personally have never built a rod with an interchangeable blank, but Fenwick makes their "Methods" rod this way, and I've seen others. I'm not sure if it's simply a friction fitting.
For me, this makes little sense, because I like to have my rods strung and ready to fish when I'm on the ice. And it seems like the blanks would have to be very similar for it to work - ie, a longer vs shorter blank with the same butt diameter.