I just installed my Pixel-Trade alien hood and, of course, I needed customized servos. I simply hate how the servo look like and I wanted some cool robots instead. I also like variety. I would love to have more than one model of servo and with default replacements "you can only have one" at time. So I just used an old technique that I discovered years ago by accident.

This is how the servo at the right becomes the servo at the left:
1. You will need: robotic skin and invisible clothes. MTS2 have some nice skins, this one in the picture is from there (search for "robot"). You can find invisible clothes at SexySims2.
2. Create a family of 2 sims: one adult male and one female. Those will be your models. Just create them. You don't need to play them if you don't want nor even add them in a lot. Just let them in the family bin. Don't delete them in-game! This will make your hood broke!
3. Create your servo. Or more than one.
4. Now close the game and open the hood at SIMPE. Use the tool Sim Surgery to make the magic. The servo will be the patient sim and the sim that you created with the robot skin will be the archetype sim. Your servo will now look like the robot sim that you made. If you don't know how to perform sim surgery,
check this tutorial.
5. You can change the sim you made with the robotic skin using SIMPE or simsurgery too. You just need to edit the line inside the sim corresponding to the sim skin to the regular default skin value. There is a tutorial to how to do it
here.
With this method you get a pair of spare sims, that is true, but you can repeat the process with different models and have several different servos in your hood with different appearances - which I personally consider an advantage.
I never waiste the 2 extra sims. I use them as archetypes over and over. I change them to use different skins with SIMPE and make facial surgery (inside the game) to change their faces to have different servos. So, IMHO is not a problem to have 2 extra sims in the hood. They are useful.
I hope you find this tutorial useful for you!