Unfortunately, creating completely new items is not possible at the moment. After the next patch, you should be able to modify an existing weapon type for one of the bows to give it an area effect attack (because the #aoe command will be fixed), then mod one of the existing bows to use whatever paths you wanted to give it for forging.
It will replace the item you chose, making it unavailable in the modded game, but it's possible to do it that way.
I tried to modify an non-existent item by number (Afaik, currently only the IDs up to 314 are used).
Well, it first seamed to have no effect and later made the game crash.
But maybe you could add a few (like: a hundred. )items there that are just placeholders for the modders to modify.