Hi,
You can get a TGraphAsymmErrors from TEfficiency after having drawn the class, via the method GetPaintedGraph. Otherwise you can yourself fill an histogram from TEfficiency by setting the content for each bin (via SetBinContent), but there is now way to set an asymmetric error in the TH1 class.
Probably we would need to improve this, we need to think about the right solution,
Best Regards
Lorenzo