A top-level White House group is expected to endorse today temporarily dry-docking two Poseidon nuclear submarines for as long as a year rather than ordering them dismantled in May as required by the SALT II treaty, according to informed sources.

The National Security Council planning session, chaired by President Reagan, also plans to discuss Air Force modification of the more than 120 B52H bombers permitted by the treaty to carry air-launched cruise missiles, sources said. The additional planes could be converted quickly to carry the missiles, they said.

The meeting is expected to prepare the way for Reagan to announce by April his long-promised "proportional responses" to what he has described for three years as Soviet violations of the unratified SALT II pact and other arms-control agreements.

Also to be discussed, the sources said, are controversial steps proposed by the Defense Department that would break the treaty's limits. These include replacing single-warhead Minuteman II missiles with multi-warhead Minuteman IIs.

Reagan's decision will be described as having been taken "more in sorrow than anger," one administration official said. He added that neither the dry-docking nor B52H modifications would put the United States irrevocably in violation and would be reversed "if the Soviets came into compliance."

Under a plan that appears to have support throughout the normally argumentative arms-control community, the submarines could be refurbished and returned to patrols after one year, depending on the arms-control situation and Soviet missile levels at that point, sources said.

If progress had been made, however, they could be reconfigured as training boats or covert transportation for special forces missions.

In May, a new Trident submarine, the USS Nevada, is to begin sea trials, and its 24 multi-warhead nuclear missiles will put the United States 22 missiles over the SALT II limit for such weapons.

In June, the Air Force will reach the SALT II limit of 120 B52s equipped to carry air-launched cruise missiles. Under the treaty, additional B52s modified to carry the missiles would qualify as equal to multi-warhead missiles and require a reduction in that category.

Last June, when for the first time introduction of a Trident threatened to breach the SALT II limit, Reagan surprised aides by ordering the dismantling of an older Poseidon, the USS Sam Rayburn. They had urged a dry-dock approach then, but Reagan agreed to "go the extra mile" to stay within SALT II and encourage a similar Soviet response.

Today's meeting may settle for the rest of Reagan's tenure the question of adhering to SALT II. The next time the United States would exceed its limits would be September 1988 when another Trident is to go on sea trials.

Thus, said one congressional backer of SALT II, "there has been a concerted push by the Arms Control and Disarmament Agency and the Defense Department to overthrow the SALT II regime."

Another new element working on Reagan is the decision by Senate Majority Leader Robert J. Dole (R-Kan.) to join 25 other senators who traditionally oppose SALT II in signing a March 12 letter to Reagan in which they urged him "not to dismantle" the Poseidons submarines "in order to comply with this 'fatally flawed,' unratified, expired treaty." Until now, Dole supported the SALT II limits.

Those pushing the president to continue adherence to the treaty limits point to Moscow's past record of destroying older sub-launched and land-based missile systems as new ones become operational.

Since 1978, the Soviets have dismantled 14 of their Yankee-class missile-launching subs as newer, more capable Delta submarines were introduced. They also modified some Yankees to be cruise-missile carriers, a costly step, but one permitted by the treaty.

To remain within SALT II missile limits, the Soviets have destroyed 70 SS11 ICBM silos as they have deployed new SS25 mobile missiles, according to the Congressional Research Service.

Based on U.S. projections of new Soviet missile deployments, Moscow to comply with SALT II, would have to dismantle 316 land- or sea-based missiles over the next two years while the United States would only have to destroy 58 for both nations to remain within the SALT II limits, according to Sen. Dale Bumpers (D-Ark.), a leading SALT II supporter of the treaty limits.

In a related action, the Soviet news agency Novosti reported that Soviet leader Mikhail Gorbachev's moratorium on nuclear weapons tests will remain in force until next Monday, despite the U.S. nuclear test last weekend.