CNC zone user stephenmhall published his DIY upgrade of Sieg Mini CNC mill. He made a custom printhead with Stepstruder MK7 extruder and Gnexlab controller.
Detailed CNC zone forum post:
http://www.cnczone.com/forums/general_3d_printer_discussion/155401-adding_3d_printing_sieg_mini_mill.html
Detailed CNC zone forum post:
http://www.cnczone.com/forums/general_3d_printer_discussion/155401-adding_3d_printing_sieg_mini_mill.html
here are two more CNC 3d printing upgrades projects: