In an organizational change that introduces a new and relatively unknown player into the administration's startegic arms planning game at a crucial moment, the National Security Council is about to create a new office to take charge of long term planning on tactical nuclear forces and strategic doctrine.

The decision by President Carter's national security affairs adviser, Zbigniew Brzezinski, to add an office for strategic planning to the NSC's staff apparatus on Sept. 15 reflects hope within the administration that the current deadlock in strategic arms limitation talks (SALT) can be broken, according to senior officials who disclosed the change. The current negotiations involve intercontinental strike forces.

The next round of talks, to be known as SALT III, are to center on reductions of the tactical nuclear weapons each superpower has targeted on Europe, and may prove to be even more complex and volatile than the two previous rounds. West Germany and other U.S. allies are deeply concerned about the prospect of the Carter administration trading off tactical advantages in Europe to obtain strategic gains.

Heading the NSC strategic planning unit will be Fritz W. Ermarth, 37, a strategic arms analyst whose work is virtually unknown to the European defense specialists who closely scrutinize all moves leading toward SALT III preparations.

Even Carter administration officials working in this area know little about Ermarth's views, since he has spent most of his career at the Rand Corp. and at the Central Intelligence Agency producing highly clasified analyses on SALT and on strategic systems in general.

Ermarth, who worked in the office of strategic evaluation at the CIA from 1973 to 1977, is currently working on a study of Air Force systems at Rand in Santa Monica, Calif.

Sources who have worked with Ermarth on Strategic planning describe him as an energetic and determined analyst who will make an impact on the increasingly open debate within the Carter administration over arms control and deployment goals. He is expected to reinforce those on the NSC who stress building up weapons systems to demonstrate American firmness toward the Soviet Union.

Ermarth's new office, which will be known as a "cluster," as are all other NSC staff operations, will absorb some of the duties of Samuel P. Huntington, a former NSC staffer who returned to Harvard University last month. Ermarth will be in charge of periodically updating the net assessment of the balance of U.S. and Soviet forces that Huntington wrote into the policy document known as Presidential Review Memorandum Ten.

Brzezinski is also putting him in charge of broad, longterm thinking about SALT III, with emphasis on Mutual Balance of Force Reduction negotiations and "gray areas," or theater nuclear force reductions, a senior official said. The official stressed that NSC would make a major effort to develop strategic doctrine in those areas.

Ermarth's appointment is being made during a period of intense discussion in Washington of theater nuclear forces and their role in SALT. The National Security Council has within the past month produced a presidential review memorandum on that subject, seeking to strike a balance on tactical weapons deployment and arms control that would please U.S. allies, according to informed sources.

Moreover, United States and the Soviet Union have to come to a general agreement on negotiated principles for SALT III before they can sign the joint draft treaty for SALT II. The principles for the third set of negotiations will be mentioned in the three-tiered SALT II agreement that may now be in its final stages of negotiations.

The subject of SALT III is expected to come up at least briefly during the talks that Paul C. Warnke, the administration's chief SALT negotiator is to hold in Moscow today and tomorrow U.S. officials said.

These officials declined to say if the NSC move to focus on the role of tactical nuclear weapons in SALT and the trip by Warnke indicate that the SALT II negotiators are within reach of an agreement that could be sent to the U.S. Senate by the end of the year, as other U.S. officials have suggested in recent days.