You also might find that TurboCAD gives you the answer that you are expecting if you make your initial object be a totally closed solid rather than an open surface fragment.

Right now from the result that you posted it seems that the thing you don't like is that it's not doing extensions on the border edges of your initial piece. If you have a closed solid there won't be any fixed-in-place border edges like you have with that open surface object and that will probably make TC extend all the different pieces in the result until they meet at the kind of structure that you want, if that is indeed what you want. If what you want instead is a 90 degree side wall between the pieces then like I wrote above the current TC result that you don't like is I think the right result for that case.

- Michael