Admin adjustablem order without cancelling old one

We have customers who place an order with us over the span of a week, constantly adding items to their order throughout the week, and finally we clear the order and ship it.  The current invoicing system means every time we want to adjust the order, we have to cancel the previous order and create a new order (which is very sluggish).  It would be nice that on the Admin side these items are just added to the invoice (yes removed from stock) and sit there until the order is finalized.  More than one of the same item may even be added by adjusting the quantity.  If something is removed from the order, it would then go back into stock.  No real need to kill and create 30 different sub orders over the week.