Actually it likely does hurt giving nitrogen to nitrogen fixers. If there is plenty of nitrogen present they reject the nitrifier association so as to not have to give up carbs in the exchange. So if you do this you will likely have to continue adding nitrogen for the few months the legumes are growing...but it is possible the legumes will be bigger, more numerous and sweeter since all of their carbs will go to their needs rather than sharing with the nitrifiers.