A senior Republican on the congressional committees investigating the Iran-contra affair will attach an addendum to the panels' report Tuesday charging that illegal acts may have been committed by key figures in the scandal.

Rep. William S. Broomfield (R-Mich.) says in his statement that a "range of potential improprieties and illegalities" were committed by former national security advisers John M. Poindexter and Robert C. McFarlane, former CIA director William J. Casey and fired White House aide Lt. Col. Oliver L. North.

Extensive excerpts of Broomfield's 14-page draft statement were made available to United Press International. The statement will be made public Tuesday along with the joint report of the House and Senate committees.

Broomfield, ranking Republican on the House Foreign Affairs Committee, says in his statement that he believes President Reagan did not violate the law, although he committed "policy errors" and "did not pursue the details of implementation sufficiently."

Broomfield, who cosigned along with seven other Republican members of the panels a minority dissent to the report to be issued Tuesday, steers a middle course between the committees' report and the dissent.

The majority report, which was signed by all 15 Democrats and three Republican senators, reportedly says that Reagan's aides and the president himself may have repeatedly violated the law in carrying out the secret arms transfers to Iran and the Nicaraguan rebels, panel members have said.

The minority dissent criticizes Reagan and his aides for errors in policy, management and political judgment but maintains that the administration basically complied with the law.