May I suggest using your postprocessor for a generic mill output?
You may have better luck saving your file as an ISO format and doing a few minor manually edited changes.
You can check that the ISO format in your 426 is enabled (if it isn't already) by going into
the file editing screen, and then bring up the program manager where the soft key row (2nd row) will allow you to select allowable file types. *.I should be shown.
If not, I can send you the parameter procedure to allow the I file extension which will let you load a regular gcode file.
This will probably work better than trying to post into a conversational format. Graphics screens, tool nose configuration, datum offsets etc will all still work in ISO on the 426, so don't let that dissuade you.
Let me know,