This is described in the PLECS manual in the sections Using Workspace Variables in Parameter Expressions (page 51) and Customizing the Circuit Block (page 47, pages refer to the 4.3 edition of the manual). In short, you need to edit the Simulink mask of the Circuit block and define your desired workspace variables as mask parameters.
This is demonstrated in the attached model and m-file. Choose Mask > Edit Mask... from the context menu of the Circuit block and open the Parameters & Dialog tab to see how an Inductance parameter was added. Choose Mask > Mask Parameters... to open the dialog to edit the mask parameters.
Notice that the model uses the PreLoadFcn callback to define a default value for the inductance variable varL in the base workspace. The m-file is a function that defines a local variable varL in the function workspace and uses the sim command to run simulations in the function workspace.