Hello Jimmy,
Unfortunately, currently it not possible to do that, and at least adding landfill as a road-tile to standard configuration will not happen. However...
Requests to add or tweak tile priorities for driving assistant is a very frequent request, and I think it's time I sit down and add some configuration options for this.
The idea would be to allow the player to specify overrides for tiles for different categories that are provided by the mod - and this should let you add the landfill as a road tile as well.
Can't make promises about when it will be done, but hopefully within a week or so :)
Best regards,
Branko