The General Accounting Office has reported finding more smoke surrounding the third White House Conference on Aging held here last year, but still not the fire that Reagan administration critics insist was there.

According to a study prepared at the request of the chairman of the House Committee on Aging, Rep. Claude Pepper (D-Fla.), the membership of key committees at the conference was changed to seat more delegates "favorable" to the administration and to remove some "unfavorable" delegates.

These changes were made, according to the GAO, after the Republican National Committee polled most delegates to determine how they felt about potential conference issues and political questions.

According to Pepper, the GAO report "confirms that the Republican National Committee sought to manipulate and control the proceedings and results" of the conference. Pepper criticized "key administration and RNC officials [who] have tried to stonewall the GAO investigation by refusing to cooperate with GAO."

The congressional auditors said that their study is incomplete because several key people would not meet with them or declined to discuss certain questions and because some documents and information could not be authenticated.

When delegates were polled by a survey firm several weeks before the December, 1981, conference, critics charged that the RNC was trying to stack the committees and manipulate the conference. Richard Richards, chairman of the RNC, appeared before a hastily convened meeting of Pepper's committee and denied the charges.

Richards said the poll was nothing more than an attempt to determine if delegates were familiar with President Reagan's positions.

Richards and Betty Brake, the executive director of the conference after the removal of David Rust, told Pepper's committee that the poll results would not influence committee assignments.

The GAO report said that committee assignments for the 2,000-plus delegates initially were made by a computer programmed to consider a delegate's state, sex and race. After the RNC poll, 815 assignments to key committees were changed, giving the administration a comfortable edge, according to documents the GAO obtained from a former conference employe.

Brake and other conference officials denied receiving any information from the RNC "relating to conference activities," the GAO said.

Additionally, the study found that the firm of Paul Manafort, a political consultant who became involved in conference activities, received $19,500 from the RNC's "White House support division fund" in March, 1982, for unspecified services.

The GAO said Mansfort told investigators that the money was for services in connection with the New Jersey gubernatorial campaign last year.

But information from the New Jersey Election Law Enforcement Commission and "the former treasurer of the campaign committee in question," the GAO said, "did not corroborate Mr. Manafort's statement. We made repeated unsuccessful attempts to reach Mr. Manafort to discuss further the $19,500 payment."

Pamela G. Bailey, assistant secretary for public affairs at the Health and Human Services Department, which was involved with the conference, said the "pre-election preliminary and incomplete" report contained "little new."