I'm not designing a PCB. I'm designing an Arduino-driven display. I need symbols for the external devices that I'm connecting to the Arduino board. Switches and LEDs I've found in the libraries; servos and power supplies aren't.
What are you using Eagle for?
Eagle is great when you want to design a circuit with the purpose of building a circuit board. Typically, you run your design through some CAM processor, and send the resulting output files to some contractor that will make your PCBs, and perhaps populate those PCBs with components.
As a second artifact, you will also get schematics that document your design. You can use this to communicate concepts to other people, without the physical device being included.
If you are using Eagle as a "drawing program" to create those schematics, rather than to produce PCB files, then the first thing you can do is to just add in text to represent the objects. The second thing you can do is create your own library. Create a library file in your eagle folder, then "use" that library, then "open" that library. Now you can select packages/symbols/devices by name, and if you type in a name that does not exist in this library, you're asked to create it. You can then draw into a drawing surface as much as you want, including adding pins for inputs/outputs, and areas where names and values will go. There are tutorials for this (google something like "eagle custom library") that will make it easy.
If you're using Eagle to make PCBs, you really don't want to solder in the servos on the PCB. Thus, you want some connection to the PCB. This means you search for "header" in the parts library, and select a header of the appropriate pitch/size/mechanical layout, and use that part. Label it "Servo A" or "Speaker C" or whatever, and use that in the design. In the schematic, you'll see a connector symbol, not a servo (or speaker) symbol, but that's OK -- you're designing a PCB, not a complete system.