The bees will draw out the foundation according to the honey flow not advailable space, a heavy flow means the bees need somewhere to put the honey so they draw out the foundation. A light flow or no flow will cause the bees to backfill around the brood nest. Stacking an empty box with foundation on top of the other supers is the best. The bees will use it if needed, placing it under drawn and full supers is possible but will confuse the hive.