Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Radius
HAIR_TOOL_ADDROOT_RADIUSParameter: Step
HAIR_TOOL_ADDROOT_STEPParameter: Count
HAIR_TOOL_ADDROOT_COUNTParameter: Length
HAIR_TOOL_ADDROOT_LENGTHParameter: Min Spacing
HAIR_TOOL_ADDROOT_MINSPACINGParameter: Pressure
HAIR_TOOL_ADDROOT_PENParameter: Interpolated
HAIR_TOOL_ADDROOT_INTERPOLATEParameter: Strength
HAIR_TOOL_ADDROOT_INTERPOLATE_STRENGTH