House-Senate conferees refused last night to deny abortion funding under health plans for federal workers and thereby add yet another restriction to unprecedented abortion curbs approved by both houses last month.

The conferees also dropped a proposal aimed at ending government pressure for affirmative action programs to aid the advancement of women and minorities.

They did so in giving tentative approval to a huge package of supplementary appropriations and spending cutbacks for financing the government through the rest of fiscal 1981, which ends Sept. 30.

The measure, which appeared to add about $20 million to spending authority for the fiscal year, is subject to adjustments if final computations indicate it exceeds the recently approved budget ceiling of $661.4 billion.

The measure is expected to be approved by both houses this week, presumably in advance of Friday's deadline for another round of stop-gap funding for many government agencies that are operating without normal appropriations. Funding for these agencies through Sept. 30 is also included in the bill.

The biggest share of the additional spending will go for defense, although the conferees shaved the huge increase for the Pentagon that was voted by the Senate to bring it more in line with a smaller but still large increase approved by the House.

The rejected anti-abortion language, which had been approved by the House but not by the Senate, would have banned the use of federal funds in connection with any newly negotiated health insurance plans for federal workers.

Its rejection came as a surprise in light of the antiabortion fervor in Congress that culminated last month in Senate approval, for the first time, of language banning Midicaid-financed abortions except when a woman's life is in jeopardy.

Existing law also permits Medicaid abortions in cases of rape or incest.

Because the House had already approved the new Medicaid abortion language, it was not in dispute in the conference and will become law, at least for the rest of the fiscal year, once the appropriations bill is approved and signed by the president. The only abortion issue in dispute at the conference was the question of federal's workers' health plans.

The Senate had rejected the House proposal on the health plans in part because it wouldn't have permitted abortions even when a woman's life was endangered. House conferees proposed making an exception to save a woman's life, but Senate Appropriations Committee Chairman Mark O. Hatfield (R-Ore.) continued to object that such legislative "riders" have no place on appropriations bills.

"We're not legislating on abortion," complained Hatfield, who has always favored legislative restrictions on abortions. "We're legislating labor-management relations," he added, noting that federal workers as well as taxpayers contribute to financing the health insurance plans.

The language banning use of the federal funds for regulations or guidelines dealing with quotas and affirmative action programs was also proposed by the House and rejected by the Senate. The conferees dropped the provision without debate after senators insisted they would not accept it.

The conferees also agreed to go along with the Senate in adding $538 million to the $1.2 million in extra money for the food stamp program to keep payments flowing until the end of the fiscal year.The Department of Agriculture had said that the 23 million food stamp recipients would face a 17 percent cutback in benefits this summer without the extra money.

On the MX missile, conferees agreed not to require an environmental impact statement if President Reagan decides to go ahead with the system. As a caveat to the two houses' agreement to equip a plant for production of nerve gas weaponry, they agreed to a proposal from Hatfield to require cost studies and consultation with allies before a decision is made to resume nerve gas production.