Ah, the file that is "smskin" is a skin file that is already packaged.
How to use an SMSKIN file:
1: put it somewhere you can find it, perhaps make a skin folder in the StarMade folder or somewhere you can navigate to it easily again.
2. Click the StarMade launcher, Click "start game", the Connection Setup launcher will appear.
3. Go to "Modding" near the top of the Connection Setup window, click that and a drop down menu appears, select "Load Custom Skin"
4. Click browse if what you want is not there, find that smskin file, put it there, hit "ok" and play as normal.
This should then load in the already packaged skin file.
(I've been giving png files for people to package on their own, this allows people access to my files, to more easily make their own versions if they wish. The smskin file is already packaged, and available to use much more quickly. The person who made the skin has to decide which they want to give to people, as they are pros and cons to both. ^_^) Hope it helps!