Armageddon wrote:learn UnrealED, it's just a lot harder.
Perhaps from your own experience. UnrealED is very user friendly and far more flexible than hammer is, it's what you see is what you get, not what you see is a pile of shit.
In response to the original question, you can export brush geometry from hammer in .dxf format from the file menu. You can then open this up in something like 3ds max, and then export to be used in unreal as a static mesh. You can then convert that static mesh to a builderbrush, which you can then create a BSP/CSG chunk from. Only do it if you are desperate though, the results are messy. Take the time to learn basic CSG manipulation in Unreal, it will be well worth it.