Like the question says, can I manipulate normals explicitly?
The reason I'm asking is because I'm making a game and I want to make some objects that fit together with other objects as they are placed by the engine. For instance, I want to model the dug-up sand besides a hole and want it to transition naturally into the (flat) ground around it, or model a skirt for the character model and want it to fit together with that character's torso.
However, when doing that the "naïve" way, the normals are (of course) calculated as if the object existed in isolation, which means that they point in a different direction from the surface they're supposed to fit together with in the end, creating a sharp edge in the lighting of the compound object, which is a bit disturbing.
Is there any way I can just "set" the normals in a particular direction? Like, straight upwards for the normals on the outer vertices of the mentioned sand, or straight outwards for vertices along the upper edge of the mentioned skirt.