Me again... I think there isn't any solution to my problem:
I found this:
http://msdn2.microsoft.com/en-us/lib...ffice.11).aspx

There's no direct way to return the series or trendline corresponding to the legend entry.

After legend entries have been deleted, the only way to restore them is to remove and recreate the legend that contained them by setting the HasLegend property for the chart to False and then back to True.