In the NPC's data in the mod tools, you should see a field something like "body data" or similar. If this is left blank, it is randomly generated for each new game. If you give it a setting (more on that in a sec), it will use that always in any new game for any player.

What you need to do is: go into the Character Creator, start a new game. Create the exact appearance (race, gender, all sliders) exactly how you want them. Then, look on the left for an "Export" command.
BEFORE YOU CLICK EXPORT, MAKE SURE TO CHANGE THE CHARACTER NAME OTHERWISE IT WILL EXPORT AS "WANDERER" LOL.
Then click Export and it will save as <charactername>.bod2
Bail on the new game you just started: just hit ESC and then choose Exit.

Now here's the thing: by default, the file will be saved in
<YourKenshiDirectory>\data\character\bodies\export
but this does you no good - an NPC will not load a file from Export! It must be in the "bodies" directory exactly. Go find the file you just saved, and COPY it (so we have a backup in case Bad Things happen) to the one-level-above-it folder,
<YourKenshiDataFolder>\data\character\bodies

This will work for you if you want to test it now: but in order to get this to other people and make it part of your mod: you must make sure that your local mod directory includes a "character\bodies" hierarchy, and place the .bod2 file in there, so it's packed up as part of your upload to Steam Workshop or whatever and will be merged/visible for the players who download your mod. In other words, you already have
<YourKenshiDirectory>\mods\MyModImWorkingOn

and must add
<YourKenshiDirectory>\mods\MyModImWorkingOn\character\bodies\TheFileWeJustMade.bod2

Go back into your mod editor, get your NPC's record, and populate that appearance field with the file we just made and copied.
Make sure to choose the file IN YOUR MOD FOLDER, so it will work for other people once you publish the mod.
Load game, test and see that it's working.

and that's it =)