Thank you for the comment. I know about this issue but at the moment I don't know how to solve it. The problem is that such a series doesn't fit into the internal data structure which is used to represent trigonometric series. The "general" coefficient is expected to be a single equation which is used to derrive all the remaining sin(n*x) or cos(n*x) terms. This makes easy to Add two series. If there would be a possibility how to specify different values for this "general" coefficient dependent on the value of "k" (e.g. using IF statement), it would be very difficult to Add this series to another series which is not fully "compatible" (doesn't have the same conditions for the value of "general" coefficient).
Please, let me know If you have any suggestions how to deal with this problem.