Hey,
to get the item_proto you need to unpack the locale , I guess you know that - Now to unpack the item_proto you can use the FA unpacker from this Topic

After you downloaded the FA unpacker extract is to C:\
Now the item_proto needs to be in the FA folder - Than pull the ex_item_proto.xml on the m2.exe - you will get a new file named item_proto_dump.xml , thats the file which you want to edit - after you finished simply pack the item_proto again (pull pack_item_proto.xml on the m2.exe)