Since they order stuff from the closest provider (import/manufacture station) I have placed the manufacture station between the importer and the car slots. I also keep the import networks separated.
If my design is done right the closest provider in at least 9/10 cases is the manufactering station.
You can do this already now. If you updated the game that is.
You can click your station and select an option to only allow local so that station will not request any parts from the import anymore. Or you can select it to prefer local, this way that station will only use the importer if there is no items locally available.
Ah, when it comes to stockpiles that idea was opted many times and I believe Cliff is working on implementing it.
A work around you can use now is to now have said stockpile connected to a line that is connected to an importer.
So you have a line from importer to the manufacture station and then have a separate line from the manufacturing station to the stockpile. This way the stockpile is completely unable to draw items from the importer.