The National Association of Manufacturers yesterday urged President Reagan to change a longstanding administration policy and join in efforts at next month's Bonn summit to improve the world's exchange rate system.

Reagan already intends to press leaders of Japan, Canada and European nations at their economic summit May 2-4 to set in motion a new round of international talks to lower trade barriers.

NAM President Alexander Trowbridge, at a breakfast for reporters, said the summit meeting should give equally high priority to launching efforts to improve the present system of flexible exchange rates among currencies, and urged Reagan to support this.

Other industrial country leaders, especially French President Francois Mitterrand, have long argued that sharply fluctuating exchange rates -- for example, the recent soaring dollar -- have been undermining the world's trading system and that nations must address this problem along with trade.

The Reagan administration has long opposed efforts to change the exchange rate system. It argues that exchange rates are determined by the fundamental economic policies that nations follow and that if anything needs to be changed it is these national policies, not the system itself.

The NAM exchange rate proposal was made in a letter to the State Department, which had solicited views of various groups on summit issues.

The letter urged the administration, as a start, to agree with the other summit nations that their central banks undertake coordinated, joint intervention to "cap" the dollar, preventing a further rise. Nations intervene by selling strong currencies and buying weak ones in exchange markets.

The NAM stressed that such joint action would assist, not substitute for action on the domestic front. Before it could bear fruit, NAM officials said, Congress must reduce the federal budget deficit which has helped push up the dollar.

The administration traditionally has taken the position that central bank intervention cannot alter long-term currency trends and should only be used in rare cases of "disorderly" exchange markets, such as might follow an important bank failure or other crisis.

However, the administration did agree at a meeting of finance ministers of five industrial nations Jan. 17 to join such coordinated intervention when it would be "helpful."

Meanwhile, in the currency markets yesterday, the dollar edged down in slow trading after Federal Reserve Chairman Paul Volcker said U.S. economic growth could soon slow down. Gold prices were virtually motionless.

Volcker told a financial conference in Washington on Tuesday that America's economic boom -- a big factor behind the dollar's record-shattering climb at the start of this year -- could peter out if some sectors of industry continued to lag.

He also reiterated calls for cuts in the U.S. budget and trade deficits, saying current levels of red ink are not sustainable.

The remarks were seen as mildly bearish by investors, and today the dollar fell back in a market otherwise devoid of trends, dealers said.

At midmorning yesterday, one dollar bought: 3.1422 West German marks, down from 3.1650 late Tuesday; 2.6615 Swiss francs, down from 2.6685; 9.6000 French francs, down from 9.6100; 3.5495 Dutch guilders, down from 3.5740; 2,000.50 Italian lire, down from 2,006.00; and 1.3740 Canadian dollars, down from 1.3725.

In London, the British pound was quoted at $1.2058, compared with $1.1987 Tuesday.

In Tokyo, where trading ends before Europe's business day begins, the dollar rose to a closing 254.90 yen from Tuesday's 254.35. Later, in London, it was quoted at 254.70 yen.

Gold opened in London at a bid price of $323.30 a troy ounce, compared with late Tuesday's $324. At midmorning yesterday, the city's five major bullion dealers fixed a recommended price of $323.30, same as the opening.

In Zurich, the bid price was $322.80, up from $322.75 late Tuesday.

In Hong Kong, gold fell 86 cents to close at a bid $323.14.

In New York Tuesday, gold fell 90 cents to close at $324.10.

Silver was quoted in London yesterday at a bid price of $6.57 a troy ounce, down from Tuesday's $6.605.