Well, took a bit of tinkering, but looking back it was quite easy, although I couldn't figure out this supposed "lookup command" for items.
Basically the item names expected by the command are not the ones displayed in the game interface. Sometimes they match, sometimes they don't. The names used by the command are those found in the \strategic\configs\stratgame\equipment\equipment.txt file extracted from gamedata.vfs (see the guide to modding for beginners for instructions).
This file has a tree-like structure for all items, and for each one there is a 'name STR' line, which simply is the name we are looking for.
bottom line :
1. "vfstool.exe se gamedata.vfs strategic\configs\stratgame\equipment equipment" or download it
2. look for "name STR" strings in equipment.txt
3. add_item and enjoy
id INT 15
name STR "M82A1 Barrett"
equipslot STR "Hand"
sortcode STR "Firearm"
hit_points INT 100
building STR "Basic human factory"
>add_item "M82A1 Barrett"