When you import a file into Maya, you can merge the display layers or render layers from the imported file into your current scene. You can merge layers of the same name or of the same index number.
For example, if you choose to merge display layers by name, then all objects on the layer named MyLayer of the imported file will be merged with objects on the layer named MyLayer of your current scene. The merged layer will retain its name MyLayer.
To merge display/render layers by layer name
Click on the Display/Render radio button in the Render Layer Editor to view the desired layers.
To change the layer name, double-click the layer for the Edit Layer dialog box to appear.
To merge display/render layers by layer number
Click on the Display/Render radio button in the Render Layer Editor to view the desired layers.
To edit the layer index number, right-click the desired layer and select Attributes from the right-mouse menu to display the display/render layer’s Attribute Editor. Enter the layer number in the Number attribute.