[valky]

I dunno if new items/ships only be available if you start a new game, but also had slightly difficulties adding some stuff into a running game new game worked fine by me...

My slightly tweaked Butcher Mk. II corvette / step by step

edit your Carcasses.xml (i took the "Butcher_pl0" , these are usualy used only by player!) Bold stuff is very important ! otherwise it won't work (you can buy it but you can't use it)
Other things are bit tweaked down...i think having a 5000 armor ship for the player 'd be too easy. slightly more speed and ~25 more maneuver / less shields & armor / sound from corvette added, should be in the file too (oh used different descriptions, so it won't work. it's only some example for whose who want to know^^)

<Interceptor name="Butcher_pl0">
<short_name>#M_Name_Butcher2</short_name>
<hint>#M_Hint_Butcher2</hint>
<short_desc>#M_SDesc_Butcher2</short_desc>
<long_desc>#M_LDesc_Butcher2</long_desc>
<mesh_name>butcher_kfni</mesh_name>
<flat_image>butcher_kfni</flat_image>
<hit_points>2500</hit_points>
<mass>2250</mass>
<disable_trade/>
<cost>300000</cost>
<technology/>
<EPR>3</EPR>
<explosion_script>Fighter</explosion_script>
<work_sound/>
<silence/>
<max_energy>1000</max_energy>
<energy_restore>3</energy_restore>
<max_speed>6.6</max_speed>
<maneurability>4</maneurability>
<steering_power>260</steering_power>
<sensor_resolution>0.38</sensor_resolution>
<sensor_length>135</sensor_length>
<threat>5</threat>
<mapping_name>butcher_corvette</mapping_name>
<HolderProperties>
<big_guns>4</big_guns>
<small_guns/>
<rockets/>
<systems>2</systems>
<turrets>1</turrets>
<gk_guns/>
<gk_turrets/>
</HolderProperties>
<engine_sound>Engine-Class3.wav</engine_sound>
<engine_start_sound>Start_engine_2a.wav</engine_start_sound>
<level>5</level>
</Interceptor>

Next step would be to define, that your fine ship also can use turrets *yay* !! (seems like they only made it possible to add a few, but nevermind thereafter you can use them all *eg*)

Modules.xml -> either manually or with a serious editor, as you don't WANT to replace all of the lines 'cause it will mess with all modules.
edit:

<TurretModule...>
<attach_type>ONLY_BIGSHIP</attach_type> <- change in <attach_type>ALL_SHIPS</attach_type>
</TurretModule>

Now that you have your corvette capable of using all turrets, you sure want to buy it

"...\Star Wolves 3 - Civil War\Data\Scripts\include\FloodTradeStations.script "

__All_ships ={......,"Trident_mk2_black_pl0","Butcher_pl0"};

the line is very important for the shop script where you can buy and so on! (in my case it would be at the end of line 5)

"...\Star Wolves 3 - Civil War\Data\Scripts\InitTradeSystem.script"

now add exactly at the position where your ship/module or whatever is another entry in _every_ local NShop*

in my case
local NShop_SI_sst = { ....

2,0,0,0,0,0,2}}; -- 5 level ships

I only made them available @MSF blackmarket shops. In every other shop it looks like:
0,0,0,0,0,0,0}}; -- 5 level ships

Hmm that's it. If you start a new (!) game you should be able to buy a Butcher Mk.II . You can edit in a running game stats like armor/shields and so on but afaik adding new items just won't work as these are init-scripts for starting a new game.
Basically you could add stuff from SW2 doing this way...most of the meshes and so on are still there.