The Reagan administration has "modified its stand" on intervention in currency markets as a result of decisions taken at the economic summit that concluded yesterday, a senior official told reporters here today before returning to Washington.

He left the impression that American policy will be somewhat more flexible than the current guideline of intervening only when U.S. officials conclude that markets are disorderly.

The official would not say directly that the modified stand would lead to greater American intervention to stabilize the rate of the dollar in foreign currency markets.

"We all know that intervention over the long term and with billions of dollars is not productive if you're fighting a major trend that has developed in currencies," he said. He said a fluctuation in currency values that "starts going into the 2 percent area" can be considered "a real spike, and accordingly you wonder, 'What's this all about?' "

The official said that, as directed by the Williamsburg Declaration, "We'll call each other up, we'll talk more about it, we'll discuss whether or not there should be intervention. Now, whether that leads to more intervention, how can you tell?"

This soul-searching over on the intervention question stems from the new commitment of the seven nations at Williamsburg to try to keep fiscal, monetary and jobs policies closely aligned.

If all of the other major nations' finance ministers believe there should be intervention--or even if most of them do--the United States might be disposed to go along, the official said.

He acknowledged that a continuing problem is that "the judgment of what is 'disorderly' changes when several people talk about it." For example, because of the present French anxiety about the depreciating franc, "they think it's disorderly if it goes from 7.50 to 7.54" to the dollar, he said. "But if it goes suddenly from 7.50 to 7.70, we would want to know if that's in line with the underlying trend, or something that should be smoothed out."

Now that the major nations seem to be on the threshold of actual convergence of their economies, their currencies will stay more closely aligned, he said. If "the currencies over the long term should be stabilizing more because the underlying economies are similar or parallel, then we will need more cooperation to make sure that random fluctuations or erratic markets don't change the perception of that" convergence, he added.

But when asked if there would have been more intervention last year had the new guidelines been in effect, he said "I honestly don't know, because I don't know how many phone calls I would have gotten where people would have insisted that 'this is a disorderly market, and you should get in there.' "

The official also hoisted one more signal that the administration is not anxious to reappoint Federal Reserve Chairman Paul A. Volcker, whose term runs out in August.

The official said that most Fed officials "don't understand" why the basic money supply has risen by about $15 billion over the last-reported four-week period. The Fed thought it had smoothed out seasonal variations, he said.

"If this money supply continues during June to be still on the high side, obviously some more tightening is going to be needed," he said.

Interest rates need not follow the money supply "if it is seen that the Fed is still an inflation fighter and that the administration is encouraging the Fed to be an inflation fighter," he said. "I think that's what the market needs reassurance on, that we're not just turning our back on the inflation fight , mouthing things and not meaning it."

But when asked if the administration couldn't give the clearest signal to the world that it wants the Fed to be an inflation fighter by reappointing Volcker, the official said: "No, I wouldn't say that.