The United States plans to play only a limited role in setting up any negotiations to resolve the civil war in El Salvador, President Reagan's nominee for special envoy to Central America said yesterday.

Former senator Richard B. Stone told the Senate Foreign Relations Committee that his mission to Central America "is a supportive mission rather than a negotiating mission." The Salvadoran government's recently established peace commission, in concert with governments of other democratic nations in Central America, will be in charge, he added.

"The minute we start to say I think they should do this first or that second, we poach on their lives and aspirations," he said. "They've had it above the nose with people telling them what to do. They are sovereign. They are proud. It's really up to them."

The modest role for the United States envisioned by Stone contrasts with moves in Congress to force El Salvador to hold unconditional negotiations with the leftist guerrillas there, by attaching conditions to further U.S. military aid. The Reagan administration has asked for an additional $110 million in military aid to El Salvador this year, about four times what has been approved so far.

Stone is expected to be confirmed by the committee next week and by the full Senate shortly afterward. By tradition, former senators are shoo-ins as nominees, but, polite as the questioning was during the hearing, several senators made clear that they were confused as to what Stone plans to do in Central America.

"Do you have some priorities?" asked Sen. Nancy Landon Kassebaum (R-Kan.), pointing out that in the case of the Middle East, special envoy Philip C. Habib has objectives that are "more clearly defined."

Stone replied that the United States "would not seek to impose our footprints or our agenda on the Salvadoran peace commission's efforts. We're not going to try to induce them to include us."

His reply prompted Chairman Charles H. Percy (R-Ill.) to ask, "I wonder if you could define your mission . . . ? "

The appointment of a special envoy to Central America was the idea of Rep. Clarence D. Long (D-Md.), chairman of the House Appropriations subcommittee on foreign operations.

Last month Long made the appointment a condition for approving $30 million in additional military aid to El Salvador for fiscal 1983, in the hopes that a "Habib-type negotiator," as he put it, would bring warring parties to the table.

The Salvadoran government, backed by the Reagan administration, has so far refused to endorse negotiations with the opposition, unless talks are limited to rules for the scheduled elections there. The guerrillas refuse to negotiate under those conditions.

Stone was pessimistic as to whether he could do anything to resolve the problem. "This is going to be a very difficult mission," he said. "I am not an optimist in this regard. The odds are so long against this kind of effort coming to fruition."

Asked if he would be willing to talk to Guillermo Ungo, a leader of the Salvadoran opposition, Stone said Ungo should talk to the peace commission, "but I would be glad to have a discussion with him."

As a Florida Democrat, Stone served in the Senate from 1975 to 1981, and was known for representing the anti-communist views of his Cuban constituents. "It is going to be difficult going into this job with such a hard-line image," Sen. Claiborne Pell (D-R.I.) told Stone, adding that the situation in Central America will be hard to resolve until the United States establishes normal relations with Cuba.

After the Senate, Stone worked as a registered agent of the rightist Guatemalan government at a time when Guatemala was seeking to lift the arms embargo imposed as a result of its human rights violations.

Stone has said he was working on the dispute between Guatemala and Belize. However, the Council on Hemispheric Affairs (COHA), a labor- and church-supported group, said yesterday that his records filed with the Justice Department show no contacts with diplomats dealing with Belize, but repeated contacts with Pentagon officials.

"It is simply scandalous this nomination didn't get the scrutiny it deserves," said COHA director Larry Birns. "Because of the old-boy network, the Senate committee did a roll-over."