Ok first you'll need Freelancer SDK witch you can get
here

Next goto C:/program files/mircosoft games/freelancer/data/equipment and open up "select_equip.ini"

Then goto the bottom and add something like this:

[Armor ;<-close this with another one Like this {Armor} only with a square one
nickname = your_armor_name_here ;<- this can be anything you want
ids_name = 263737 ;<- this don't touch this unless you know what your doing
ids_info = 264737 ;<- same as above
hit_pts_scale = 2.000000 ; <-this is what the your armor is multiplied by 1 means no armor change, 0.9 and lower decreases your armor, and 1.1 and higher increases your armor.
material_library = equipment\models\hardware.mat ;<- don't change this unless you know what your doing
DA_archetype = equipment\models\hardware\br_protective_armor.3db ; <- same as above
volume = 0.000000 ; <- change this to make it take up cargo
mass = 10 ; <- doesn't effect ship as far as I know
lootable = false ; <-you can have to options here true witch means you can loot it and false mean it can't be looted

Then in the equipment folder open up misc_equip.ini and put this at the bottom:
[Good ;<- close with another like {Good} but with a square one
nickname = your_armor_name_here ;<- must be same as the one in select_equip
equipment = your_armor_name_here ;<- same as above
category = equipment ;<- no idea don't change
price = 100000 ;<- do i really have to explian this one
item_icon = equipment\models\commodities\nn_icons\EQUIPICON_armor.3db;<-don't change unless you know what your doing
combinable = false ;<- i think this one means if you can mount more than one at a time but don't change
ids_name = 263737; <- don't change unless you know what your doing
ids_info = 264737;<- same as above
shop_archetype = equipment\models\hardware\br_protective_armor.3db ;<- don't change unless you know what your doing
material_library = equipment\models\hardware.mat; Same as above

Then in the equipment folder open up market_misc.ini and find the base you want to add it to and paste this this below it
MarketGood = your_armor_name_here, 1, -1, 10, 10, 0, 1

Here's a link to a file with alot of good info:
Freelancer Miscellaneous DataBase

Note you need a program that can view .xls files
You can get one here
Excel Viewer 2003