it seems that it is not possible. I had also posted the same question and Chris answered to me. See link http://hec-ras-help.1091112.n5.nabble.com/Saving-maps-in-shapefile-tp5295.html
Thanks jarvus. I wanted to make sure there was not a more correct way to calculate a weir. It could be a good idea to impose a rating curve, previously calculated.