We present the concept and design of a nonlinear frequency division multiplexed transmission system in which a broadband nonlinear spectrum is stitched together with help of a photonic integrated circuit. Four independently modulated WDM channels are linearly multiplexed together with trapezoidal filtering and partially overlapping spectra, approximating a seamless wideband nonlinear spectrum in which guard bands are avoided. We show that this approach mitigates the problem of nonlinear interaction between the channels, enabling more efficient use of the available spectrum. The system is highly scalable and allows for add-drop multiplexing using optic-electronic-optic signal processing. Different forms of nonlinear spectrum modulation are discussed and the system is simulated to establish its performance limits. A data rate of 400 Gbps is shown to be supported over 800 km with single-polarization modulation and a spectral efficiency of 4 b/s/Hz, with a BER staying below the SD-FEC limit for every subcarrier.