To me it seems like the PBNI is incomplete when it comes to IPB_RSItemData, since there seems to be no way to create the interface.

But you are able to directly call the PowerBuilder methods of a datastore using something like:

    pbobject datastore;

    PBObjectFunctionInvoker insertrow(m_Session, datastore, L"insertrow");
    insertrow.GetArg(0)->SetLong(1);
    insertrow.Invoke();

    PBObjectFunctionInvoker setitem(m_Session, datastore, L"setitem");
    setitem.GetArg(0)->SetLong(1); // row
    setitem.GetArg(1)->SetInt(1); // col
    setitem.GetArg(2)->SetDouble(13.37);
    setitem.Invoke();

    PBObjectFunctionInvoker getitemnumber(m_Session, datastore, L"getitemnumber");
    getitemnumber.GetArg(0)->SetLong(1); // row
    getitemnumber.GetArg(1)->SetInt(1); // col
    getitemnumber.Invoke();

    double ret = getitemnumber.GetReturnValue()->GetDouble();