How to add sims to world w/o having to play them?

@Bhroseph  If you go to Manage Worlds within an ongoing save, you can create new sims, or download them from the Gallery, and place them in houses in your existing save without having to play them.  Once they're placed, just click on the household you've been playing, and resume where you left off.

You can also place households you've saved to the Library, so if you create a new save and want to import some of those sims to a different save, you can save them to the library and place them in the new save via Manage Worlds.

If you start a new save from the Main Menu, your existing sims won't be in it.  Each save is its own world, starting from scratch, as in, the EA premades are there in their default states, but none of the sims you've created will be unless you add them manually.

@Bhroseph Just to add to what @puzzlezaddict wrote, if you'd like to fill some of the more expensive homes and the NPC households do not have enough funds to buy them, you can use the cheat called freerealestate on. This cheat will unlock all homes, even ones the NPC households cannot normally afford.

@Bhroseph 
In addition to the suggestions by @puzzlezaddict  and @RandomBuzziness    let me add another option.
I made a tutorial that still works.  My friend also made a video to demonstrate how to follow the tutorial.

Tutorial: Moving Sims In From the Gallery Without Money Cheats
https://forums.thesims.com/en_US/discussion/920585/tutorial-moving-sims-in-from-the-gallery-without-money-cheats/p1