The one OSTexo links to above (http://moi3d.com/forum/index.php?webtag=MOI&msg=4388.55) is usually the way you would do thing with regular NURBS modeling tools - you need to split it in two and blend between the pieces. Also check out here for a Rhino video of the technique, you can do the same steps in MoI: https://vimeo.com/3297459

But creating branching structures is one of the things that tends to be a more natural fit for sub-d modeling techniques than NURBS modeling techniques so especially if you are going to be doing lots of branches the sub-d method could be good to use instead.

- Michael