How do people go about creating new importers/exporters?

Hi i mainly model things for gaming purposes (mods) and most of the time by the time i have found the game someone has already created an importer and exporter plugin that can be used with its filetype. However i am now attempting to help a small community create a importer and exporter plugin for the game "battlestations midway" which keeps its model files in the form of ".mmod". All i currently know is some vague information that the files are based on the lightwave 3d format, but i can not open .mmod files with standard lightwave 3d importers. Basically, i am wonderinf if anyone else knows how you create the .dll importer and exporter files. Does it take a lot of skill? some other program i do not have? what is the process generally used?

Thank you.

Google didnt help? To create your own exporter you should know a good programming skills