А как присвоить Null, ведь Convert.dbnull здесь не проходит?

Ну параметр можно просто не присваивать или присвоить ему значение DBNull.Value

            Dim paramFields As New ParameterFields()
            Dim paramFieldDt As New ParameterField()
            Dim paramFieldOper As New ParameterField()
            Dim discreteValDt As New ParameterDiscreteValue()
            Dim discreteValOper As New ParameterDiscreteValue()
            Dim CR_R As New ReportDocument()
            CR_R.Load(Application.StartupPath.ToString + "\Reports\CR_AggrFullOtchKassira.rpt")

            paramFieldDt.ParameterFieldName = "@Dt"
            paramFieldOper.ParameterFieldName = "@Oper"
            paramFieldOper.EnableNullValue = True
            discreteValDt.Value = Me.DateTimePicker1.Value
            paramFieldDt.CurrentValues.Add(discreteValDt)
            'discreteValOper.Value = DBNull.Value
            'paramFieldOper.CurrentValues.Add("22")
            'paramFields.Add(paramFieldOper)

            Me.CrystalReportViewer1.ParameterFieldInfo = paramFields
            Dim logOnInfo As New TableLogOnInfo()
            Dim i As Integer

            Dim paramFieldDefDt As ParameterFieldDefinition = CR_R.DataDefinition.ParameterFields.Item("@Dt")
            Dim paramFieldDefOper As ParameterFieldDefinition = CR_R.DataDefinition.ParameterFields.Item("@Oper")
            Dim ParameterValuesDt As ParameterValues = paramFieldDt.CurrentValues
            paramFieldDefDt.ApplyCurrentValues(ParameterValuesDt)
            Dim ParameterValuesOper As ParameterValues = paramFieldOper.CurrentValues
            paramFieldDefOper.ApplyCurrentValues(ParameterValuesOper)