How to add objects to a neighborhood common area?

You would probably need something like TwistedMexi's T.O.O.L. (take objects off lot) mod for that: https://www.patreon.com/posts/t-o-o-l-11-15-31592360 if it's off lot. Otherwise, the dumpster is a gameplay object in Build/Buy so you can add it to a lot.