President Reagan and Soviet leader Mikhail Gorbachev have agreed to add a joint appearance Thursday at the end of their meetings in Geneva, where they are likely to sign cultural and air-safety agreements and to review the summit, White House officials said yesterday.

In another development, the State Department last night said that the Soviets had resolved 10 of the 25 U.S.-Soviet cases involving separated spouses, dual nationals and divided families. This equals the number of such cases settled in the past two years, and was termed by a State Department official as "a signal prior to the summit" of Soviet willingness to resolve the problems.

These developments came as Reagan prepared to depart this morning for Geneva and the first superpower summit since 1979. Sources said that Reagan and Gorbachev are expected to agree to establish a regular process of consultation, including future summit conferences. Story on Page A10.

A senior White House official said that the Thursday ceremony that has been added to the summit schedule also could include either joint or separate statements by the two leaders on summit accomplishments.

A White House official said it is also possibile that the ceremony will include a statement of instructions to arms control negotiators who will resume talks in Geneva on Jan. 16. Other officials said these instructions could be a joint statement or in the form of separate statements.

Reagan will fly to Geneva from Andrews Air Force Base today after a departure ceremony at the White House. He is scheduled to participate in a brief arrival ceremony tonight, then rest and review briefing papers for two days before the summit.

White House chief of staff Donald T. Regan said that Reagan "feels that he is ready" for his first meeting with a Soviet leader. Regan said that the one-on-one meeting of Reagan and Gorbachev that is scheduled to open the summit Tuesday would be an "icebreaker" that would enable each leader to take measure of the other.

Regan said that opening the summit in that way will give the two leaders "a better opportunity when they get in and start to talk to be able to look at each other eye to eye, having seen each other 15 or 10 minutes they're more familiar with each other."

Officials said both countries were "working very hard" in an effort to complete work on a new cultural accord, which Reagan said in a nationally televised speech Thursday night was near agreement, and on a Pacific air safety agreement involving the United States, the Soviet Union and Japan.

The air safety talks were prompted by the Soviet downing of a South Korean airliner on Sept. 1, 1983. An agreement would open the way for resumption of U.S.-Soviet commercial air service that was canceled at that time. Negotiations on the air safety treaty continued last night in Washington, and U.S. officials said they had extended the visas of the Soviet bargainers.

Describing the Thursday ceremony, a senior official said: "As we see it now, the two leaders in some public forum would sign the documents and would each make a statement. Ours would probably be on how we saw the summit. Then there would be some more casual conversation between the two, and they'd leave."

The senior official said that Gorbachev is "the head of the Communist Party and a very staunch advocate of his cause" and that Reagan is under no illusions that the meetings will be easy.

"You cannot expect him to be soft, you cannot expect him to be genial, you cannot expect him to be anything except what he is, leader of the Soviet people and a very dynamic person," the official said.

In an extensive and unusual effort to inform allies of the results of the summit, the assistant secretaries of state for African, East Asian, Latin American and near Eastern affairs will fly to Geneva for meetings with top U.S. officials Wednesday and then fan out to tell leaders of important countries what happened between Reagan and Gorbachev.

Reagan spent his last day in Washington in an hour-long National Security Council meeting reviewing summit themes. An official said the president's advisers "don't want to overload him" with briefing material and added that Reagan was rereading earlier papers that had been given him.

Reagan also met during the day with Sens. Pete Wilson (R-Calif.), and Ernest F. Hollings (D-S.C.).

They presented a petition signed by 37 colleagues urging Reagan not to agree to restrictions on his Strategic Defense Initiative (SDI). The senators said they shared Reagan's view "that SDI is too important to be traded for marginal improvements in the status quo."

"The quest for a world free of 'push button' Armageddon must not be abandoned for short-term gains in the superpower thermostat," the senators said. "Ironically, we have let the Soviets make real progress on their campaign against our SDI, while they proceed apace on their own."

Earlier this week, Reagan was briefed on the Soviet Union by three CIA analysts.