During World War II, the Nazi regime did a great deal to exploit the resources of the countries that they counquered. One might argue that they could have incorporated those countries' resources more efficiently, but it is clear that the Germans tried hard to extract productive resources from the countries they conquered.
The most obvious way in which the Nazis did this was through the use of slave labor or, at least, of coerced labor. Over the course of the war, Germany forced millions of people from conquered countries to come to Germany to work. The link below shows us that, by 1944, there were 7 million people from foreign countries working in Germany. Essentially none of these people were free workers. In this way, Germany tried to take the labor force of many of the occupied countries and use those resources for German ends.
The Nazis also did a relatively systematic looting of the conquered areas. Large amounts of machinery, among many other items, were simply taken from the conquered lands and brought back to Germany. This represented another attempt to incorporate those countries' resources into the German war effort.
It is possible that the conquered nations would have been more cooperative if Germany had been less brutal in its exploitation. This might have made Germany better able to incorporate their economies into its own. Be that as it may, Germany clearly tried hard to exploit the resources of the conquered countries.