Key House members said yesterday that they will seek to overturn the Reagan administration's budget recommendations and provide additional federal funds next year to expand the Metro subway system.

President Reagan has proposed halting all federal spending for Metro construction after the current fiscal year. The move has jeopardized the transit authority's plan to extend the 60.5-mile rail system to more than 80 miles by the early 1990s.

Rep. William Lehman (D-Fla.), chairman of the transportation subcommittee of the House Appropriations Committee, which plays a key role in allocating transit aid, said yesterday that he expected the panel to recommend further funds for Metro. "I think Metro has taken on a life of its own," Lehman said.

Rep. Lawrence Coughlin (R-Pa.), the subcommittee's ranking Republican, and several other subcommittee members expressed similar views. None of the subcommittee members, however, proposed a specific spending level for Metro.

At the same time, Ralph L. Stanley, the Reagan administration's mass transit chief, signaled tentative support for a new stopgap proposal by Metro officials to expand the system to 84.7 miles by 1992. "In principle, we think the plan meets the requirements," he said.

The Metro proposal was drawn up in an attempt to persuade the administration to release nearly $400 million in federal funds appropriated by Congress in the last two years. The Metro board has not taken final action on the plan, and Stanley said the issue would require further federal review.

Stanley testified as the House subcommittee began hearings on the administration's proposals for sharp cuts in federal spending for mass transit systems here and across the nation. The administration's recommendations are expected to encounter stiff opposition in Congress.

In an initial move, the Senate Budget Committee voted last month to reject the administration's overall proposals for transit spending, including the cutoff of Metro funds. The Senate committee's plan allots $217.2 million for Metro construction next year, the same level as the current appropriation.

Asked whether the House subcommittee may recommend a similar spending level for Metro, Coughlin replied, "I think that's plausible." Metro General Manager Carmen E. Turner urged the subcommittee to endorse a $250 million appropriation, $25 million less than the authority's initial request.

At issue are Metro's plans to complete two major sections of the long-delayed Green Line in the District and Prince George's County, along with a Red Line extension to Wheaton and a Yellow Line spur to a Van Dorn Street station in Alexandria's West End.