The White House is expected to designate an experienced diplomat to work with Richard B. Stone, the president's newly named special envoy to Central America, in an effort to defuse congressional criticism of the appointment, administration and Capitol Hill sources said yesterday.

"The proposal is under active consideration," said an administration official.

Administration sources said the idea arose in a conversation with aides of Senate Majority Leader Howard H. Baker Jr. (R-Tenn.), who at one point suggested that Reagan should name three special envoys to the region in an effort to secure a negotiated settlement.

Stone, a former Democratic senator from Florida who subsequently served as a lobbyist for the right-wing government of Guatemala, was appointed to the new post Thursday by the president, who called the action a demonstration of "bipartisanship" in foreign policy.

However, administration sources acknowledged yesterday that the appointment did not sit well with Democratic leaders in the House, including Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), and had failed to accomplish the president's purpose of winning additional Democratic support for the military aid funds requested for El Salvador.

These sources said the president's appointment of Stone would be "strengthened" by addition of a career diplomat of the stature of Walter J. Stoessel Jr., who retired last fall as deputy secretary of state and is respected in Congress and the diplomatic community.

Though Stone is expected to be confirmed by the Senate, which traditionally ratifies former members, some administration officials said yesterday that the appointment would be more popular if he were backed up by a professional diplomat.

The most frequent criticism of Stone on Capitol Hill is that he lacks the diplomatic experience or credentials to carry out a delicate negotiation that would aim at finding a political solution to the military conflict between the government and leftist guerrillas in El Salvador.

"We're looking to bring more people into the policy consensus," said one official.

"If it would help to have a diplomat as an associate of Stone or a White House consultant or counselor on Central America, we'll give the idea serious consideration."

A high administration official said the concept had been discussed with the president but that no names had been submitted to him. Reagan was said by one official to be strongly committed to doing "whatever is necessary within reason" to build bipartisan support for his policies in Central America.

Presumbably, the designation of a career diplomat also would improve the chances of bringing the left-wing insurgents in El Salvador into discussions with government representatives. One of the concerns voiced on Capitol Hill by moderates of both parties is that Stone will lack the trust of the rebels because of his outspoken support for right-wing governments in the region.

However, officials emphasized that Stone would be consulted before any associate would be named and that the White House wanted to be careful not to undermine efforts of the new envoy before he has even begun his work.