SimPowerSystems™ software provides a variety of basic building blocks to build more complex electric blocks. Using the masking feature of the Simulink® software, you can assemble several elementary blocks from the powerlib library into a subsystem, build your own parameter dialog box, create the block icon that you want, and place this new block in your personal library.
Building and Customizing SimPowerSystems Models describes how to build a nonlinear model using a Voltage Measurement block and a Controlled Current Source block. The proposed examples (a nonlinear inductance and a nonlinear resistance) are relatively simple. Using the same principle, you can develop much more complex models using several controlled current sources, or even controlled voltage sources.