Subdivide your mesh in edit mode or cut the mesh with the function knife.
the Modifier Bevel can help you too (in object mode).
You can also create your door in highpoly and then create a normalmap to use it on a lowpoly version in the game.
The Modifier - Multiresolution is used for this, it allows to work on a highpoly vesion of the model while keeping the lowpoly version. once the baking of normalmap realized, just apply it on the lowpoly version