Ah well.
Yes, renaming the file extension on a file should do it.
BUT
Easiest thing? Copy an existing file that already has the coding you want.
I recommend you dissect my mod to help you learn. For example, your going to have make a shipdef file... I did so and reorganized the listing of ships within to be more naturally flowing, as you have to do a lot of edits in that file.
Was thinking of making a video tutorial on shipsets soon. I'll let you know!