You can just change the file extension although the way that your post processor created the file (g-code) it will most likely not work. All you really have to do is cut out the first few lines of mill settings code in the nc file (up to "n1") and replace that with the setup code from a known working ncd code. also you might want to go to the beginning and end and make sure that the %(stop start prog) codes are gone as ncd format usually use "(start)" and "(end)". also you want to make sure that the feed rates etc. are all want you want them to be. This can all be done fairly simply with a text editor like notepad, or if there is a lot of changes you wish to make you can import them into excel.