The only way I know is making a family with kids. Have the kids get taken away by the Social Worker. They are then placed into the adoption pool.
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You CAN make a family with kids and then make the kids get taken away by the social worker, but I don't think it's neccesary to go to all that trouble.....because there's a much faster and easier way to "seed" the adoption pool! Here's what you do:
-Go into CAS (create-a-sim) and create a bunch of adult sims, making sure you don't make them related to each other (as otherwise they will still recognize each other as relatives even after being adopted). Give each one a name you like or at the very least can live with (since they will still have that name after being adopted).
-Age each one down to toddler (or child, if you'd prefer to seed the adoption pool with children instead of toddlers) to make sure they look human and at least half-way decent rather than something from another planet as toddlers or children (beautiful, CAS-created adults don't always look the best as toddlers/children!). If they DO look bad as a child/toddler, tweak their appearence until they do. I also recommend not using any CC make-up on the sims, as most of it looks quite awful on toddlers, and not much better on children. Then age them back to adult and save the group (I say group and not family since they aren't related to each other).
-Grab the group out of the family bin and plop them on a lot (I recommend putting an empty lot in your neighborhood to use for this purpose).
-Enter the lot the group is on. Once the lot has loaded, bring up the cheat window and type in the following: agesimscheat on
-Then age the sims down to toddler or child. Once you are done, turn off the cheat by typing the following: agesimscheat off (thus preventing yourself from forgetting it's on and accidentally aging a sim you didn't want to, as I once did!).
-The social worker should arrive within about five sim seconds and haul those toddlers/children off to the orphanage. Presto! Adoption pool seeded!