What is the problem?
you add it your new item into the next free line and use an item number which is between the item number of the first item and the item number of the second item.
If you customized your item numbers with an interval of 1, then you cannot get a 3rd item between 1 and 2, but if you have an interval of 10, then you just add number 15 and you get the 3rd item between 10 and 20.