I wouldn't know whether this is allowed or not, as I never had use for such a service, but I know a few people myself who used an expediter in Brazil ("despachante") to ease the process of getting a U.S. visa.
Clearly I know very little.
- go to the embassy's web page,
- load all your personal information for the visa application in the web page.
- schedule an interview
- schedule the visit to the CAS (where they take you a photo and your fingerprints).
So, basically, they manage the bureaucratic procedures that happen BEFORE you get to the interview, and give you advice on the documentation you should take with you to prove you are economically rooted to your country (so you won't become an illegal immigrant)...
But you still need to go personally to both the CAS and the Embassy (on 2 different days). So, not really worth it for anyone who knows how to use a webpage.