You can't have sims in your Library appear in the game directly. They have to added to the game manually, just like any household you play. To do that, just pick a household in your Library and move it onto a lot. If you don't want them occupying a house, just evict them so they are "Not in World" just like the NPCs. One thing to remember is aging. So, if you have aging on for NPCs, they will eventually die off. If you play with aging off for NPCs, then that's not a problem. And you will have to decide what features of the Neighborhood Stories feature you want active for them.
They will show in Manage Households under the Other tab.
Hope this helps.
MC Command Center had this as an option in MC Population : Tray Import Percent.
You can find MCCC in the hero approved mods post on the bug forum.
