frxRep.Variables['A'] := 'a';
frxRep.Variables['B'] := 'b';
frxRep.Variables['C'] := 'c';
...
frxRep.ShowReport;

for i:=10 to Dataset.FieldsCount - 1 do
    if ( fDS.DataSet.Fields.FindField(VarName)<>nil ) then
      Value := fds.DataSet.FieldByName(VarName).Value;

unit frxClasses.pas
...
function TfrxReport.DoGetValue(const Expr: String; var Value: Variant): Boolean;
var
  i: Integer;
  ds: TfrxDataSet;
  fld: String;
  val: Variant;
  v: TfsCustomVariable;
begin
  Result := False;
  Value := Null;

  if Assigned(frxFR2Events.OnGetValue) then
  begin
    TVarData(val).VType := varEmpty;
    frxFR2Events.OnGetValue(Expr, val);
    if TVarData(val).VType <> varEmpty then
    begin
      Value := val;
      Result := True;
      Exit;
    end;
  end;

  { maybe it's a dataset/field? }
  GetDataSetAndField(Expr, ds, fld);
  if (ds <> nil) and (fld <> '') then
  begin
    Value := ds.Value[fld];
    if FEngineOptions.ConvertNulls and (Value = Null) then
      case ds.FieldType[fld] of
        fftNumeric:
          Value := 0;
        fftString:
          Value := '';
        fftBoolean:
          Value := False;
      end;
    Result := True;
    Exit;
  end;

  { searching in the sys variables }
  i := FSysVariables.IndexOf(Expr);
  if i <> -1 then
  begin
    case i of
      0: Value := FEngine.StartDate;  { Date }
      1: Value := FEngine.StartTime;  { Time }
      2: Value := FPreviewPages.GetLogicalPageNo; { Page }
      3: Value := FPreviewPages.CurPage + 1;  { Page# }
      4: Value := FPreviewPages.GetLogicalTotalPages;  { TotalPages }
      5: Value := FEngine.TotalPages;  { TotalPages# }
      6: Value := FEngine.CurLine;  { Line }
      7: Value := FEngine.CurLineThrough; { Line# }
      8: Value := frxGlobalVariables['CopyName0'];
    end;
    Result := True;
    Exit;
  end;

  { value supplied by OnGetValue event }
  TVarData(val).VType := varEmpty;
  if Assigned(FOnGetValue) then
    FOnGetValue(Expr, val);
  if Assigned(FOnNewGetValue) then
    FOnNewGetValue(Self, Expr, val);
  if TVarData(val).VType <> varEmpty then
  begin
    Value := val;
    Result := True;
    Exit;
  end;

  { searching in the variables }
  i := FVariables.IndexOf(Expr);
  if i <> -1 then
  begin
    val := FVariables.Items[i].Value;
    if (TVarData(val).VType = varString) or (TVarData(val).VType = varOleStr){$IFDEF Delphi12} or (TVarData(val).VType = varUString){$ENDIF} then
    begin
      if Pos(#13#10, val) <> 0 then     //<< вот тут возникает
        Value := val
      else
        Value := Calc(val);
    end
    else
      Value := val;
    Result := True;
    Exit;
  end;

  { searching in the global variables }
  i := frxGlobalVariables.IndexOf(Expr);
  if i <> -1 then
  begin
    Value := frxGlobalVariables.Items[i].Value;
    Result := True;
    Exit;
  end;

  if not Assigned(frxFR2Events.OnGetScriptValue) then
  begin
    { searching in the script }
    v := FScript.FindLocal(Expr);
    if (v <> nil) and
      not ((v is TfsProcVariable) or (v is TfsMethodHelper)) then
    begin
      Value := v.Value;
      Result := True;
      Exit;
    end;
  end;
end;

Value := Calc(val);

frxRep.Variables['A'] := #39 + 'a' + #39;
frxRep.Variables['B'] := #39 + 'b' + #39;
frxRep.Variables['C'] := #39 + 'c' + #39;