Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Shadow
CELSHADER_SHADOWParameter: Diffuse
CELSHADER_DIFFUSEParameter: Specular
CELSHADER_SPECParameter: Illumination
CELSHADER_ILLUMParameter: Use Bump
CELSHADER_BUMPParameter: Backfacing
CELSHADER_BACKFACEParameter: Camera
CELSHADER_CAMERAParameter: Lights
CELSHADER_LIGHTSParameter: Specular
CELSHADER_SPECULARParameter: Illumination
CELSHADER_ILLUMINATIONParameter: Use Light Color
CELSHADER_LIGHTSCOLParameter: Shadows
CELSHADER_SHADOWSParameter: Shadows
CELSHADER_SHADOWSTYPEParameter: Mode
CELSHADER_SHADOWSMODEParameter: Invert
CELSHADER_SHADOWSMODE_INVParameter: Use Lights
CELSHADER_INCEXCParameter: Lights
CELSHADER_INCEXC_LIGHTS