at least if engine could manage outer world.
In fact , all world is in one 3D or 4D array like skyscraper , so forest and outside world could be in any level , 3D model trees are needed for this.
Another idea is , when the player goes outside of the dungeon , before go outside the player will open one door and will see the image of the forest path outside , like as you entering in stairs and you see the sequence of climbing stairs and then insted you are on the forest level , you will see one overland map (2D Picture) and then there will be spots of dungeons you can visit , the idea works like teleports when you chose one spot you will get transfered.