Yes it can be done. I did it. It's not really very difficult at all.
It does, however, require a mod from NRass Industries called Porter which can be found
here.That mod might require the NRass mod Master Controller which can be found on the same site as porter along with the rest of his mods. I think most of his mods require Master controller.
If you've never used mods before than getting your Sims three folders set up to accept mods will be the most challenging part. Not that it's really all that difficult, it's just easy to mess up by putting something in a wrong folder somewhere.
Once you have porter installed you want to make sure you have all the households that you want to populate your world with saved to the library. Once that's done and you've exited Sims 3 then open your world in CAW then in CAW select Edit In Game from what ever menu they hid that under (I don't remember at the moment.) and once in EIG you first have to make a homeless sim by hitting Control-C to open the input line and type MakeSim. That new homeless sim will automatically become your active sim which is required to be able to click on the houses and select NRass>Porter>Unpack at which point you'll see a list of all the households in the library but you'll only see a list of the household names, not the thumbnails, so they need unique names to be able to identify them.
Once you're finished populating your world Save and Exit. It's that easy.
A couple caveats though...
It's best not to populate the world until it's the very last step before packaging.
Also I recommend avoiding unpausing in EIG at all costs. There's never really a need to unpause but if a sim dies for any reason in EIG it will likely ruin the whole world causing you to have to start over.
