Donald T. Regan, chairman of Merrill Lynch & Co., the nation's largest brokerage firm, surfaced as an added starter in the Reagan cabinet sweepstakes yesterday as transition officials suggested he may be in the running for secretary of the treasury.

Regan's named popped up on a day when Ronald Regan's chief aide, Edwin Meese III, tried to stanch the flood of Cabinet rumors, saying the rumor-mongers don't know what they're talking about. Speculation continued in Republican cricles, though, and these were among the items:

Rita Hauser, a New York lawyer and longtime acitve Republican, was mentioned as a candidate for U.S. ambassador to the United Nations. Hauser was a member of the U.S. delegation to the U.N. under President Nixon. Another person suggested as a possible U.N. ambassador was Michigan Gov. William Milliken.

Rep. Manuel Lujan (R-N.M.) went back to the top of some lists of candidates for secretary of interior. This occurred after Sen. Alan Simpson (R-Wyo.) announced that his friend Clifford Hansen, who had been rumored a top candidate, had taken himself out of consideration for the job.

Some prominent Republican businessmen were said to be protesting the reported choice of retired general Alexander Haig as secretary of state.

A top business adviser to the president-elect said that, in addition to Merrill Lynch's Regan, prominent businessmen who might be under consideration for treasury secretary include General Electric Co. Chairman Reginald H. Jones and TRW Inc. Chairman Ruben F. Mettler.

The rumor mills offered differing assessments of employment prospects for Walter Wriston, who was, a few days ago, reported to be the best bet for the treasury job. Some Republicans said Wriston, chairman of New York's Citicorp, the holding company that controls Citibank, the nation's second largest bank, was still the top choice.

Others noted that Wriston might have difficulty winning confirmation, even from a Republican-controlled Senate, because of Citicorp's involvement in several highly visible problems that will confront the new treasury secretary. Subsidiary Citibank is one of Chrysler Corp.'s creditors, for instance, and the Treasury Department oversees the government's loan guarantee program for Chrysler. Citibank is also heavily involved in the controversy over frozen Iranian assets and in federal loan guarantees for New York City.

Such problems would apparently be somewhat less troublesome for Regan, although Merrill Lynch plays an active role in domestic and international banking, real estate and insurance, as well as the securities and commodities markets.

Meese, the transition director, made fun of the Cabinet rumors yesterday. "Don't be misled by all the rumors that are going around," he said. "... Nobody who knows what's going on is talking. . . . All these people who purport to know are really not in the know."

Meese confirmed yesterday that Reagan does not intend to replace FBI Director William H. Webster. Noting that Webster has seven years of his 10-year term remaining, Meese said: "We believe the 10-year term, as for good government, is a good idea for the FBI." He added that Webster "has done a very good job, I think most people would agree."

Some of the guessing over Reagan's Cabinet may be put to rest soon, Meese indicated. He said the president-elect would announce an initial group of nominees in the next few days. Meese complained that the "rather complex" process of clearing nominees who have been settled on had delayed the announcement.

"Because of . . . the conflict-of-interest regulations and the requirements of the various committees on the Hill," Meese said, "it's no longer as it used to be where a president calls up a Cabinet member, he consults with his wife, and he calls him back in 24 hours and the deed is done.

"It's now a situation where any prospective Cabinet member . . . has to sit down, literally, with a lawyer from the transition team, with his own lawyer, his banker, his accountant . . . and it often takes as much as a week, in which he himself is figuring out how he can comply with all of the regulations. . . . "

A reporter asked jokingly if this meant Reagan had not chosen any paupers for Cabinet jobs. Meese laughed and replied: "Even paupers nowadays have to fill out the statement, you know, you have to declare the value of your food stamps and all those other things."