Cadsoft Eagle Library: How to create device variant?
In the library editor, go to your device and you'll see the traditional screen.  Here is the "rcl" library - probably the most popular library there is.



On the right side is the package variant section.  Here is where you can assign multiple packages to the same device.  You simply click on "New," select your new package, give it a useful name, and add it.  You can then associate schematic pins to footprint pins with the "Connect" button. You can see in rcl how the resistors use one schematic part but a large number of package variants.