I built a 30k+ member vbulletin community in the 2000s.

I used sock puppets with different personalities and views that created threads, replied to each other and encouraged real users to reply and made them feel welcome.

I also moderated as myself and was welcoming to real new users, (and the puppets :)

This did not last more than two months, as there were enough real users that the puppets were able to largely hibernate. But it did happen, and it worked.

I never outed my puppets and they were not used longer than necessary.

Puppets will not sustain a community, but they can help establish one.