A House-Senate conference committee boosted President Reagan's effort to save his rearmament program yesterday by giving final approval to a compromise $282.5 billion defense appropriation while providing a separate $6.3 billion dividend which could offset the automatic cuts envisioned by the Gramm-Rudman-Hollings law.

Counting the money appropriated for nuclear warheads and military construction in separate legislation, the compromise figure comes to just under $299 billion, or slightly more than $3.5 billion under the $302.5 billion Reagan sought to keep his defense program at last year's level plus inflation.

Earlier, in setting budget ceilings, the House had voted for limiting the Pentagon to $292 billion for fiscal 1986 and the Senate, $302 billion. The House went along with the $302 billion only after instructing its negotiators to drive the total back down to $292 billion in the appropriations process just completed.

During the intense bargaining behind closed doors, participants said, the House's major victory was banning additional testing of antisatellite weapons unless the Soviets break their moratorium on such tests, while the Senate's big win was raising the amount of money available to the Pentagon.

The $6.3 billion is a kitty of money left over from prior appropriations. The House had wanted these funds to be applied to the fiscal 1986 budget, but the Senate prevailed in making it an addition to the total.

The conference report will instruct the Pentagon to use most of the $6.3 billion to finance military and retirement pay, expected to be more than $4 billion, during the current fiscal year rather than follow the usual practice of submitting a supplemental budget request.

Critics of creating what one called the "$6 billion honeypot" said the Pentagon would not be bound by the conference report language, and predicted it would try use the money to take the sting out of the antideficit law known as Gramm-Rudman-Hollings after its sponsors, Sens. Phil Gramm (R-Tex.), Warren B. Rudman (R-N.H.) and Ernest F. Hollings (D-S.C.).

Rep. Les AuCoin (D-Ore.) said last night that he felt the moratorium on antisatellite tests outweighed what House conferees lost on funding levels. "Those are redeemable next year," he said, predicting Gramm-Rudman-Hollings and Congress' budget-cutting mood will bring the totals back down.

The Senate gained ground in its effort to clear the way to resume the production of nerve gas by providing the money to gear up for production. However, chemicals could not be purchased and, before production, European allies would have to assure the Pentagon that they would allow chemical weapons to be stored within their borders.

Junked during the House-Senate conference was the package of Pentagon procurement reforms, mostly crafted by the House, which lost out in the earlier process of working out a defense authorization bill, the measure which specifies how much money can be spent on ships, tanks, missiles and other weaponry.

AuCoin, a member of the House-Senate conference on the continuing resolution which included appropriations for defense, said losing the procurement reforms "was a bitter pill to swallow, especially since I upbraided Les Aspin for losing them" in the earlier conference on the authorization bill. Aspin (D-Wis.) is chairman of the House Armed Services Committee.

Participants said Rep. Joseph P. Addabbo (D-N.Y.), chairman of the House Appropriations subcommittee on defense, who is recovering from surgery, did not put up as great a fight as usual in the negotiations. Rep. William V. Chappell Jr. (D-Fla.) took over during Addabbo's absences.