Or must I re-arrange/re-code all the items according the the item.dat number?
In short, yes.
In FRUA98, each group of items has an item class number at the top. The settings in the upper half of the menus (item class, attack range, missile etc.) are the same for every item in a class. When you make changes there, it will affect all items in that class, so be careful.
In the bottom half, under "Item Class, Magic Bonuses", there is a box labelled "Item Class". Enter the target item class number there, the item will then be transferred to the target class and take on the basic properties of that class.
There are plenty of unused item.dat numbers for you to create new item classes.
But pay attention: Some of the item classes have inherent properties that can't be changed. Classes 61-63 are the Drow weapons, for example. These items will always vanish when the "destroy drow items"-option in a transfer module event is checked. Likewise, class 74 always has the blessed bolt specAb.
We discussed a lot of the specifics in
this thread and
this thread.