President Reagan, moving to avoid almost certain defeat at the onset of a critical legislative session, today will administratively impose economic sanctions against South Africa that closely parallel those in a bill that he has repeatedly threatened to veto, congressional and White House sources said yesterday.

Senate Majority Leader Robert J. Dole (R-Kan.), who predicted earlier that the sanctions bills would pass easily and that a veto could not be sustained, termed the anticipated White House package "a great step forward" and said he will ask the Senate to postpone its scheduled vote this week.

"If the president makes a strong statement and does things that are already in the legislation, we'd declare victory, all of us," Dole said on CBS-TV's "Face the Nation." "It's much better from our standpoint to have Congress and the president working together with one voice on foreign policy, that being the president's.

"If we want to play politics and square off with the president, I don't think we have the same impact on the South African government," Dole said.

The president's action would ban the sale of South African-produced krugerrand gold coins, prohibit most new bank loans to South Africa and halt the sale of computer equipment used to administer South Africa's apartheid system of racial separation, the sources said.

Reagan is also expected to announce that U.S. Ambassador Herman Nickel, recalled for "consultations" nearly three months ago as a sign of displeasure with South African policy, will return to Pretoria soon. Nickel is to carry with him what one source described as a "strong letter from the president" urging policy changes.

The sources said, however, that Reagan will significantly modify provisions in the bill that would mandate further sanctions if South Africa does not make specified progress toward dismantling apartheid. Reagan is said to want "running room" on further curbs.

The ban on krugerrand sales would be premised on approval of the General Agreement on Tariffs and Trade, the 90-nation organization based in Geneva that monitors international trade regulations.

The administration has made open markets a linchpin of its policy on the increasingly volatile foreign trade issue, the sources said, and wants to underscore that the ban on krugerrand sales is based on opposition to apartheid, not on "protectionist" sentiments.

Sources said the United States was obliged as a treaty signatory to seek GATT approval of the krugerrand sales ban. But none could say what the administration would do if GATT opposed it.

A White House showdown with the Republican-controlled Senate over sanctions has threatened from the start to cripple the administration's hopes for success in a critically important fall legislative session. Dole said yesterday that he thought the vote on sanctions now could be put off until spring.

Most measures in the pending bill, approved by the House, 380 to 48, Aug. 1, would not go into effect until 1987, Dole said. "If the administration doesn't follow through on what they suggest," he said, the Senate could bring the legislation back to the floor then.

Secretary of State George P. Shultz met with Dole late Saturday night on Capitol Hill. He briefed other key members of Congress later. It was not immediately clear, however, how much Senate support there might be for postponing a vote on sanctions.

Sen. Richard G. Lugar (R-Ind.), chairman of the Senate Foreign Relations Committee, could not be reached for comment. An aide to Sen. Nancy Landon Kassebaum (R-Kan.), chairman of the African affairs subcommittee, said Kassebaum was "caught off guard" by Dole's statements and would have no immediate comment.

Sen. Claiborne Pell (D-R.I.), ranking minority member of the Foreign Relations Committee, said of the pending action, "It's a step in the right direction, but I regret that all of the measures that both houses of Congress passed and which are supported by the vast majority of the American people are not being adopted as of now."

Reagan has consistently opposed economic sanctions, saying that they would hurt South Africa's 21 million blacks most, and has argued instead for continuation of "constructive engagement," or quiet, tolerant diplomacy, which he said Friday was "the only thing that's shown any signs of improvement in the whole situation as yet."

The proposed ban on the sale of the one-ounce krugerrand, which at one point earned South Africa $500 million a year, will be accompanied by efforts to begin U.S. minting of a similar coin.

Other components of the legislation likely to be included in the White House package are provisions that loans could only be made for health, educational and housing facilities open to all races and a requirement that U.S. firms in South Africa not discriminate in employment or living conditions.