There are two ways to do this. First you need to buy the expensive telescope, have your male sim look through the telescope any time from 7pm to 2am it may not happen the first night so just keep trying. When they get abducted there will be a cutscene with the space ship coming, your sim will be gone his motives will go crazy the next day they will drop him off again, there is a very small chance that they won’t, after they drop him off he will be pregnant it goes through just like a regular pregnancy. The baby will be an alien. You can also get a hack that will allow your female sims to get pregnant when they get abducted, but with the base game it is only the male sim that this works for.
Or if your sim is a knowledge sim you can use the lifetime aspiration points (if you have The Sims 2 Freetime) and buy all the way up to summon aliens just click the telescope and summon aliens
I hope this has helped you in getting an alien sim
The only difference between the regular sims and aliens are the aliens are green and have black eyes