Hi,
if you want to import dxf files with arcs and curves you have to follow some rules…
kicad can manage best dxf file if exported from LibreCAD in R12 version…
moreover it doesn’t support polyline
if you respect those rules, you can import very complex shapes
You may find these conversation useful:
http://www.mail-archive.com/kicad-developers@lists.launchpad.net/msg15228.html
http://www.mail-archive.com/kicad-developers@lists.launchpad.net/msg15217.html
and the kicad user manual, obviously
http://docs.kicad.org/en/pcbnew.html
at the paragraph
‘6. Create and modify a board’
‘6.1.2. Using a DXF drawing for the board outline’