The Carter administration in its final hours made six appointments to federal agencies here that some of the agencies learned about only yesterday.

The appointments apparently are not subject to challenge or abrogation by the Reagan administration, which on Wednesday "withdrew" 27 last-minute Carter nominations for judgeships and other posts that require Senate confirmation.

Unlike these nominations, which were for full-time jobs, the appointments are to federal boards and commissions that may exert considerable influence, but which meet infrequently and pay members little more than transportation costs, usually only $100 a day.

The Fine Arts Commission, watchdog of aesthetics in the nation's capital, learned only yesterday afternoon that it has two new members, Alan Novak, a Carter supporter and former aide to President John F. Kennedy, and Harold Burson, head of the New York public relations firm of Burson-Marsteller.

The Pennsylvania Avenue Development Corporation, which is supervising the rebuilding of America's "main street," has three new members -- a development that PADC officials said they were still trying to confirm yesterday: Anne Wexler, a senior aide to President Carter; Lawrence B. Simons, a Carter-appointed assistant secretary at the Department of Housing and Urban Development, and Peter B. Kelly, a Hartford, Conn., attorney.

The National Capital Planning Commission, federal planning agency for the Washington area, has a new member to replace Chairman David Childs. The appointee is another White House staffer, Bruce Kirschenbaum, who was a former assistant to the mayor of New York City who became deputy assistant to the president for intergovernmental relations.

Officials at the planning commission said yesterday they had no official knowledge of any appointment to replace Childs, whose six-year term on the agency expired Jan. 2. Besides appointing Kirschenbaum to replace Childs, Carter also apparently designated a new NCPC chairman from among the commission's 12 members. She is Helen M. Scharf, a former Montgomery County Planning Board member whom he appointed to the federal commission in 1979.

Scharf said yesterday that "I was asked Friday or Saturday" by White House official Peggy Rainwater, "but I've had no official word." Rainwater, associate director of presidential personnel and now unemployed, said late yesterday that while the Carter appointments "can't be vetoed" by the Reagan administration, the new president "can change the chairman."

The chairman plays a leading role in the frequently controversial issues that come before the commission. Last month it approved two new underground Smithsonian Institution museums on the Mall and next month it will vote on proposed changes to the regional comprehensive plan that call on Arlington to limit the height of its high-rise buildings visible from the Mall.

Former planning commission chairman Childs had asked the Carter administration to allow Reagan to appoint his replacement. Carter already had named two of the three presidential appointees on the commission and a Reagan appointment was seen as appropriate.

The two last-minute appointments to Fine Arts followed close upon three other Carter appointments to Fine Arts in November, causing an almost wholesale change in the seven-member commission that has a major say on the appearance and shape of the city. Carter earlier appointed Texas architect John Chase; Sondra Myers, a "lay" member from Pennsylvania, and Chicago architect Walter Metsch to the commission. Only landscape architect Edward D. Stone and Commission Chairman J. Carter Brown, whom Carter reappointed in November, provide any continuity for the commission.

The 27 11th-hour nominations made by Carter and withdrawn by Reagan could be resubmitted after the new administration reviews them. Carter withdrew all of former president Ford's last-minute nominations, but resubmitted a number of them later, Rainwater noted.