Adding a part does not void the warranty.
However if you add springs and a shock goes bad on you they could say the springs caused it and not cover it. If you add an intake and something happens to the motor that they can say was caused by poorly filtered air going through the intake they could deny your waranty. But if you add springs and you have an engine problem they cannot deny your waranty because of the springs.
more info at http://www.sema.org/main/semaorghome.aspx?ID=50096