For as far as I can see: it's all a matter of excruciatingly accurate 'scripting'. AFAIK a settlement plan txt file tells the engine where to place 'pieces' of wall, and thus, if you wouldn't mind extraordinary lag (because of the sheer size involved) you could actually create a settlement with multiple walls. I'm not an expert on this though, but I recall reading a thread about a tool to calculate wall outlines and such. I believe it's somewhere in the modding guides/ tools subforum.

The problem this thread originally was about still stands: can EB include a new type of wall. My guess is not, as there is apparently a dire need for 3D moddelers...