Solved, here is the solution
Joint newJoint = new Joint(); // declare a new Joint
// Iterate in the 20 Joints
foreach (JointType item in Enum.GetValues(typeof(JointType)))
{
newJoint = skToBeMoved.Joints[item];
// applying the new values to the joint
SkeletonPoint pos = new SkeletonPoint()
{
X = (float)(newJoint.Position.X + (someNumber)),
Y = (float)(newJoint.Position.Y + (someNumber)),
Z = (float)(newJoint.Position.Z + (someNumber))
};
newJoint.Position = pos;
skToBeChanged.Joints[item] = newJoint;
}
This will work.