@all4hockey54r, I haven't tried building a custom dorm myself, but I expect it will be the same way as you would get townies to move into any other home.
That is, by going into Edit Town and placing the sims you want from the library into the dorm.
This is all pre-setup by the builders of the Sims University and other EA worlds.
If you are replacing an existing dorm and want to use the sims that would have lived in that dorm, you would need to save those sims to the library before demolishing it.
Another way, if you have your custom dorm saved to the library, ready to place, would be to evict the sims from their original dorm while in Edit Town which places them temporarily in the clipboard, demolish the old dorm, place your custom dorm, and then place the sims in it from the clipboard.
To make a custom dorm function as such, set the Lot Type from Private/Residential Lot (I'm not certain which) to Apartment Lot. It's a new selection that came with UL. I don't think you even need to call roommate services, but if they don't show up for a day or two you may need to. xx
The one time I built a custom dorm, I placed in on a new lot & left the original 3 dorms where they were. When I moved my Sims in, the game moved in 4 other sims. However one of the original dorms was left empty.
I have not done this with a dorm, but when I built and placed my own Fraternity house, the game assigned seven other random male students as residents.
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That is, by going into Edit Town and placing the sims you want from the library into the dorm.
This is all pre-setup by the builders of the Sims University and other EA worlds.
If you are replacing an existing dorm and want to use the sims that would have lived in that dorm, you would need to save those sims to the library before demolishing it.
Another way, if you have your custom dorm saved to the library, ready to place, would be to evict the sims from their original dorm while in Edit Town which places them temporarily in the clipboard, demolish the old dorm, place your custom dorm, and then place the sims in it from the clipboard.