I know how to get them in right spots in Devolution V7, but not sure in any other, i bet its the same! ;D
1. You need your item ID, open up your items.cfg, add them there with item name, examine and specs like other items.
2. item.java, in there you will probably see all the ID's in RS, put the ID for a platebody under the "body" and the "platebody" spots, and "hats" under hats spot etc...
3. Then compile your server, run it, then it will work
For the weapons you dont need to do anything as all new items is at weapon spot ;D
(therfor you will only need in, items.cfg for the weapons)