Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Shell
SOLIDIFYGENERATOR_SELECTION_POLYGON_BOUNDARYParameter: Start Cap
SOLIDIFYGENERATOR_SELECTION_POLYGON_SOURCEParameter: End Cap
SOLIDIFYGENERATOR_SELECTION_POLYGON_CAPParameter: Start Shell
SOLIDIFYGENERATOR_SELECTION_EDGE_BORDER_SOURCEParameter: End Shell
SOLIDIFYGENERATOR_SELECTION_EDGE_BORDER_CAPParameter: Start Cap
SOLIDIFYGENERATOR_SELECTION_VERTEX_SOURCEParameter: End Cap
SOLIDIFYGENERATOR_SELECTION_VERTEX_CAP