Not a long process, really.
1. Use Object builder and load items.otb from newer client, then export chosen items.
2. Load 8.6 items.otb in Object builder and import chosen sprites, set properies like "movable" then save it.
3. Open itemeditor and load items.otb, reload items list, set WAREID to newly added items, save - then you got tibia.dat and tibia.spr ready and thats all. The items.otb made with object builder you use in your server items folder.
4. After that you update your items.xml with new items and its done.