Republican presidential front-runner Ronald Reagan yesterday opposed President Carter's efforts to shut off the boatlift of Cuban refugees and slap fines on U.S. boat captains who continue to make the Cuba-Florida run in private vessels.

"I can't agree with that," Reagan said in a brief meeting with reporters here. "As long as thousands are trying to get here, I can't understand the lack of humanitarianism in that," referring to the president's policy.

On Wednesday, Carter ordered a halt to the rag-tag private boatlift that has been going on for weeks but which the White House says is illegal and also increasingly dangerous. Instead, the White House wants a more orderly, U.S.-sponsored air and sealift in which the refugees are first screened on Cuban soil by U.S. officials, something to which the Cuban government of Fidel Castro is not likely to agree.

The evidence seems clear, Reagan said, that "Castro, in his own way, has started to subvert this humanitarian move by dumping undesirables from his own country here on us." And, he acknowledged the concerns in this country about U.S. unemployment.

"But if the building is burning, and there is still a chance to get people out of the upper-floor windows, you don't worry about jobs," he said.

The former California governor was in Washington to meet with three key Republican senators and to attend a three-hour session behind closed doors with his growing corps of foreign policy and defense "adviser," who now number 90.

Reagan issued a list of 67 defense and foreign policy advisers last month and yesterday added 23 names to the list, inclluding former deputy secretary of defense and NATO Ambassador Robert Ellsworth and former CIA deputy director Vernon R. Walters.

At the Capitol, Reagan met with his national campaign chairman, Sen. Paul Laxalt of Nevada; Texas Sen. John Tower, chairman of the Senate Republic Policy Committee, and Sen. Barry Goldwater of Arizona.

While they were meeting, the assistant Republican leader in the Senate, Sen. Ted Stevens of Alaska, told reporters he felt Reagan ought to announce his vice presidential choice before the last round of primaries June 3.

Stevens said Sen. Howard H. Baker Jr. of Tennessee was his first choice but he also suggested Reagan "take a good look at" Sen. Nancy Landon Kassebaum of Kansas as "a very forceful vice presidential candidate."

The defense and foreign policy strategy session brought together many specialists from around the country who could wind up in key jobs if Reagan wins the general election in November.

One clue to possible leading roles emerged when Richard V. Allen, Reagan's top national security adviser on the campaign and the organizer of yesterday's session, told reporters that the work of various groups of advisers would be coordinated by Dr. Fred C. Ikle and Laurence Silberman on foreign policy issues and by Edward L. Rowny and William Van Cleave on defense issues.

Ikle is a former director of the U.S. Arms Control and Disarmament Agency. Silberman is a former ambassador to Yugoslavia. Rowny is a retired lieutenant general who represented the Joint Chiefs of Staff in the strategic arms limitation talks with Moscow. Van Cleave is the director of the Institute for International Studies at the University of California.

Reagan heard his advisers expound on several major themes: broad U.S. defense and foreign policy strategy for the 1980s, the military balance of power, the defense budget, the Middle East, the Third World, NATO and Europe and U.S.-Soviet relations.

Allen said the idea of pulling together this group was to provide the candidate with day-to-day information on fast-breaking events and to assist him in elaborating on basic themes.

And, he added, without being "presumptuous," the group helps Reagan "examine the agenda that a new administration would face in January."