At present this is not possible. There are a number of reasons for this, the major one is that a discontinued product is no longer available with no more supplies available once you run out; this essentially means a product is either discontinued or not, it can't be both.

Discontinued products can be sold for as long as you have stock. Backorders can not be placed on discontinued stock, orders from channels will never import and new sales orders can not be placed for the item in this situation.