I don't believe it is possible. It may be possible to set up a home to seem like a business in the regular neighborhood, using visitor controllers or throwing constant parties to get people over there. As far as I know, you cannot house a controllable Sim in Downtown. Or were you thinking of setting up a store or office in Downtown and bringing your Sim there to pretend he/she owned it or worked there? I remember having a sleuth Sim, named "Guy Noir" after the Garrison Keillor stories from Lake Woebegon. He had an office, but it was out of his house. Now that you bring this up, I think I want to make one Downtown! Great idea - thanks so much for it!
Along those lines, I once set up the largest lot in the basic neighborhood to contain 4 medieval buildings with bedrooms on the second floors. My little "Villager" family had crafts that they worked at. They were tradesmen, bakers, and painters. In another neighborhood, I built a space-themed pizza place, but the pizza was free to all the visitors. When I created an elderly Dutch couple in a very narrow home, the cookies they baked by the plateful were also free to all visitors.
Addicted to The Sims since 2000.