Lewis A. Dunn, for the past two years a special assistant to Ambassador Richard T. Kennedy at the State Department, is about to be named assistant director of the Bureau of Nuclear Weapons Control at the Arms Control and Disarmament Agency, according to sources.

The job has not been filled permanently since the start of the Reagan administration. The White House's first nominee, Norman Terrell, ran afoul of Sen. Jesse Helms (R-N.C.) and was unable to win Senate confirmation.

The administration finally gave up trying to win approval of Terrell's nomination last December and named him instead to be an associate administrator for policy at NASA, a post that does not require Senate confirmation.

A former senior staff member at the Hudson Institute, Dunn joined the State Department in 1981 as a special assistant to Kennedy, who was then undersecretary of state for management.

By assisting Kennedy on nuclear issues, Dunn enabled the former Nuclear Regulatory Commission member to play a major role in formulating the administration's nonproliferation policy.

Kennedy was relieved of his management duties when George P. Shultz became secretary of state a year ago and was put in charge of nonproliferation policy with the rank of ambassador.