I hve no need for a spline to DXF convertor
so I am not going to go out and write one. However I did minimal research (wikipedia) and found that they can be simple text files, and they have a published reference guide to the structure (see references below).
What I would probably do first is to get a simple DXF file and open it up in your favorite text editor and look to see what numbers end up where.
Then I would probably replace those numbers with some placeholders
and save is at a template.
Next would be a simple program to read in that template, and replace those placeholders with the numbers I want in there.
Then I would save this as MyFirstAttempt.DXF
and see if it worked
And there would be some troubleshooting involved, but it's a start.
DXF Spec- http://images.autodesk.com/adsk/files/autocad_2012_pdf_dxf-reference_enu.pdf