To model rigid bodies with different mass parts, create a rigid body with a CompositeShape. Normally, all child objects in the CompositeShape have the same density - but: If the child of a CompositeShape is a RigidBody, the mass properties of this child body are used.

Note
The children of a CompositeShape are of the type IGeometricObject and a RigidBody implements IGeometricObject. Therefore, a rigid body can be the child of a CompositeShape.

Now, to model a hammer: Create a rigid body for the metal head. For example:

BoxShape headShape = new BoxShape(0.03f, 0.03f, 0.1f);
MassFrame headMass = MassFrame.FromShapeAndDensity(headShape, Vector3F.One, 7850, 0.01f, 3);
UniformMaterial metal = new UniformMaterial("Metal")
{
  StaticFriction = 0.6f,
  DynamicFriction = 0.6f,
  Restitution = 0.2f,
};
RigidBody head = new RigidBody(headShape, headMass, metal)
{
  Pose = new Pose(new Vector3F(0, 0.1f, 0))   // The relative hammer head position in "hammer space".
};

Create a second rigid body for the wooden shaft with different mass and different material properties:

CylinderShape shaftShape = new CylinderShape(0.015f, 0.2f);
MassFrame shaftMass = MassFrame.FromShapeAndDensity(shaftShape, Vector3F.One, 700, 0.01f, 3);
UniformMaterial wood = new UniformMaterial("Wood")
{
  StaticFriction = 0.5f,
  DynamicFriction = 0.3f,
  Restitution = 0.2f,
};
RigidBody shaft = new RigidBody(shaftShape, shaftMass, wood);

Then, add both rigid bodies to a composite shape:

CompositeShape hammerShape = new CompositeShape();
hammerShape.Children.Add(head);
hammerShape.Children.Add(shaft);

Use MassFrameFromShapeAndDensity(Shape, Vector3F, Single, Single, Int32) to compute the mass frame for the composite shape. FromShapeAndDensity automatically uses the mass properties of the child bodies:

MassFrame hammerMass = MassFrame.FromShapeAndDensity(hammerShape, Vector3F.One, 100, 0.01f, 3);

The density parameter in this method call is irrelevant because each child of the composite has pre-defined mass properties.

If the hammer uses a CompositeMaterial, the simulation will automatically use the materials of the child bodies:

CompositeMaterial compositeMaterial = new CompositeMaterial();

The first two bodies are only used to define the composite shape - they are never added to the simulation directly. Only the composite rigid body is added to the simulation:

RigidBody hammer = new RigidBody(hammerShape, hammerMass, compositeMaterial);
mySimulation.RigidBodies.Add(hammer);

For the simulation the hammer is a single rigid body. The child bodies are only used to define mass and material properties.