You can add light sources to the light source library by adding an item to a group, then loading an IES file.
To add a light source to the light source library:
Access the light source library. How to
Select the group you want to add the light source to.
Click
the Add Item button below the light source window, or right-click in the
light source window and select Add Item.
A new entry is added to the list.
Type a name for the light source and press ENTER.
Click the Load IES File button.
Locate and select the *.ies file and click Open. The filename is displayed in the right pane of the Light Sources dialog.
If you want to change the intensity of the light, specify the desired percentage in the Scale edit box. For example, if the light source is a 60 Watt bulb, a value of 50% would make the bulb function like a 30 Watt bulb.
If you want to change the color of the light, click the Color edit box and make a selection from the Color dialog.