Type in 'unlockoutfits on.' This enables you to make your alien have an 'alien' costume.
Step 1: Cheat: First, press control-shift-C before entering Create a Sim, which opens the cheat console.If you want a 'real' alien, follow this guide below: As Create a Sim in The Sims 3 comes with a tool which lets you choose an exact skin tone, you can create an 'alien' by making a Sim have green skin as well as various other alien features.
