All LayOut pattern fills are raster images so they are not directly translatable into AutoCad vector hatches. AutoCad hatches use a plain text definition file (.pat) with description of the lines (plain or dashed) that the pattern consists of.
I was chatting with Trent about this, and one thing he said was “We convert our patterns to hatches when they go out.”. That’s the part I hope to be true.
I had a try. I haven’t got AutoCad on my own computer, only TrueView so I cannot see inside blocks or try taking things apart but what I got looked like embedded clipped raster images that duplicated my fills. The same thing you get when exporting Hybrid rendered views. Definitely not a hatch.
Probably, it all depends on what result you want to get. As @Anssi said, AutoCAD does not perceive LO patterns as vector graphics, but that does not mean that it is not possible to transfer an object with a LO patch to the AutoCAD environment without losing its appearance. Only then will it be a raster patern. When exporting, a group of objects will be created from each object, where each of its subelements will be limited both by the size of the pattern module and the boundaries of the object. It is best noticed by objects made of curved lines.
Another peculiarity will be that this object is not exported to the CAD model environment but to the layot environment (in this case it will be Page1) only. You can get it into the model space with Ctrl+C - Ctrl+V.
Elements added in LayOut are on the “page surface” so naturally they are not moved inside the viewport automatically. In AutoCad, there is a command that changes paperspace objects to modelspace and vice versa.