Hi
one of program that work good for convert bitmap to gerber or DXF was excelcam
http://www.reliant-eds.com/PCB_Design/excelcam.html
demo work for convert mono bitmap to DXF and GERBER
for convert bitmap to solid monocrome you use painbrush "paint can"
for change color to black , you may generate 1 DXF for each color then combine it back
but once in pcb silk was monocrome anway ..
one converted import it into your cad ,
then in pads if my memory was good you may save pcb in ascii (.asc)
and convert it back into a part ,but i not use pads myself so i not knot exact procedure