This work introduces a new method, to the author’s knowledge, to design a freeform-mirror-based optical system from scratch by adding segments and an additional mirror. Each segment forms a stigmatic pair for a respective given field. Then, the segments of the last mirrors are concatenated with a numerical interpolation. The overall system is optimized to ensure a good performance in the fields not considered by the initial set of segments. The results are as expected by the theory and are explained with an illustrative example.