I don't think it's that difficult to produce vaguely scifi looking clothing without sculpting.
Scifi tends to rely heavily on jumpsuits, the color black and bright colors without being all that different from modern clothes. Look at Alien, Farscape and The Fifth Element for examples.
More than half the figures in the Wargames Factory survivor sets could easily fit the bill. If they have an odd action pose, give them a scifi looking do dad that makes it look like they are doing some form of manual labor.
Alot of military figures can be used without their weapons as Spaceport crew, ship crew, people in flight suits, etc. The old Void and Warzone plastics are great for this. Just need to give them more civilian looking heads.
If you want cheap aliens, start with Mantic Marauders or Forge Fathers and give them head swaps. There are alien heads available from Hasslefree, 3D Miniatures, Heresy, Blackcat Bases, and many others. Well, some of those may be demon or cthullu-esque heads, but will work. Throw in a few Tyranid arms from a friends leftovers and you're golden. GW ghoul heads can be obtained fairly cheap and could work as aliens.
It's not that hard to make longer or shorter "alien" legs by cutting parts from 2 different sets and splicing them together. This will take a little green stuff, but most of the work can be done by shaving the plastic into shape. Connections can be hidden with wrist bands, arm bands, boot tops, etc. so they don't have to be difficult or complex conversions.
There's tons of fairly simple and easy ideas out there. Given time, I'm sure I could come up with many more.