ok, gun-kata: if you have rigid bound your nurbs model, then you can not mirror skin weights because that is option is for smooth skin bind. but, if you have rigid bind then you will be able to adjust the skin weight maps by going to deform> paint cluster weights tool>options ..
if you use this tool, then in there there is an option called reflection. you can turn this on, and all the action you perform on one side of the model will be reflected on the other side depending upon the axis you choose.

epatata, i don't know about rigid skinning , but it is possible to export skin weight maps in smooth skinning through skin>edit smooth skin>export skin weight maps..