A 53-year-old man who had long-term prison sentences for heroin-trafficking reduced after he became a witness against other drug dealers was convicted in Brooklyn yesterday of conspiring to distribute heroin. A jury in Federal District Court returned the verdict after a three-day trial of the defendant, Frank Lucas. He faces not only up to 30 years in prison for that conviction, but also up to life in prison for violating the terms of his parole from the earlier sentences. In 1976 Mr. Lucas was convicted in a Federal trial in Manhattan and a state trial in New Jersey of charges stemming from his leadership of a major heroin-distribution ring. He received a total of 70 years in prison. The next year he began giving evidence that led to the convictions of more than 100 narcotics violators, and in 1981 his 40-year Federal term and 30-year state term were reduced to time served plus lifetime parole. In the new case, Mr. Lucas was charged with having conspired with an undercover agent to set up a drug sale.