I am a new GIS intern in my state's Historic Preservation Office. We have a "master" map with all of our state's archaeological sites in four layers (depending on whether it is a site, a survey area, etc.) I have some shapefiles that consultants have created for their reports (points, lines, and polygons) that I need to add to our map layers. They don't have attribute columns in common (so can't use Merge) and I have tried both Union and Intersect with no luck. I get error messages with both of these, because I don't want to create a NEW layer, I just need to add the shapefiles to the existing "master" map. I feel like this should be a very simple procedure, but perhaps I am thinking all around the solution. Any help would be appreciated.