Use Kotor Tool. First thing, you must know your item's UTI file name. Then go to the RIM section of K tool. Once there, select a module. If you are looking for a certain merchant, say Kebla Yurt, then look in Taris' modules which hold merchants. Usually it would be in Blueprint, Merchants. Select it, add the item to its inventory, and bingo, done.
For the module, look for it, and check the module with _s.rim at the end, that contains the placeables
For placeables, it is a little tougher. Especially common placeables Find the blueprint for the placeable, say a footlocker, in the Template, Blue Print Placeables section. Once you find it, delete all scripts it holds, and its inventory. Then save it with a different name, say footlocker099.utp, as if you just add it as footlocker001, it would replace all the other footlockers in the game. So, you need to edit the module's GIT file, which is in the standard module file without the _s. Look in Dynamic Area Info, and there is the GIT file. Extract it, and edit it with the seperate KGFF Editor, not the one with Kotor Tool.
Here is all the tools you will need in modding:
http://www.starwarsknights.com/tools.php)
It is a little complicated, but minimize the sections. Usually the GIT file is seperated into sections. Sound, Doors, Placeables, etc. Minimize the sections with clicking the plus or minus button next to the name until you see a section called: Placeables. Add your placeable to the GIT or replace an existing one with it and the custom name.
Now heres the tough part. You need to find the coordinates:
X, Y, Z, Bearing. Use the Whereami cheat to do this all ingame, or the whereami armband. Use the cheat ingame, and mark it down on paper. Then put the coordinates in the correct sections in the GIF editor.
Here is the organization
Name
Tag
X
Y
Z
Bearing (I think)
hope this helps