The Reagan administration plans to ask Taiwan and South Korea to allow their currencies to rise in value against the dollar, but a Treasury official said yesterday the negotiations would have little impact on the U.S. trade deficit.

A Treasury spokeswoman confirmed a Wall Street Journal story that said the administration, responding to protectionist pressures in Congress and from businesses over the Taiwanese and Korean trade surpluses, this week would request consultations on exchange rates with the two Asian countries.

But, a senior administration official said, "In terms of its ultimate impact on our trade deficit, I don't think that the outcome of exchange rate negotiations in Asia is going to be dramatic. But I think it's something that we should do, and we're planning to."

He suggested that there may be a greater possibility in winning concessions from Taiwan than from Korea, which is a large debtor nation. Taiwan has a trade surplus with the United States of about $11 billion, almost twice that of Korea.

Economist C. Fred Bergsten, director of the Institute for International Economics, recently suggested to Korean authorities that they let the won appreciate 15 percent to 20 percent against the dollar. A higher exchange rate might cut the Korean trade surplus by $1 billion to $2 billion, and the Taiwanese surplus could be reduced by as much as $5 billion, he said.

It is not known what the Treasury targets for a higher-valued Korean won and Taiwanese dollar will be. Officials refused to discuss details on any specifics, including the level of officials who would participate in the consultations.

The talks, which could get under way in August, were ordered by Treasury Secretary James A. Baker III, the spokeswoman said.

The proposed talks on Taiwan-U.S. and Korea-U.S. currency relationships are an extension of the effort made during the past year to bring down the level of the dollar against the currencies of U.S. trading partners that have a large trade surplus with the United States.

For example, the dollar has dropped at least 30 percent against the Japanese yen and the West German mark. The dollar's lower value will reduce the cost of U.S. products in those countries, making them more competitive against Japanese and German merchandise. Officials ultimately hope that the U.S. trade deficit will be reduced as U.S. exports increase. But a potential gain based on exchange-rate changes cannot be expected from many other countries, including Korea and Taiwan, if their currencies continue to be tied to the dollar.

In conversations with key Korean officials earlier this month, Bergsten suggested that a higher-priced won was in their own national interest.

Bergsten estimates that in 1986, Korea will be running the first global current and trade account surpluses in its modern history -- representing a dramatic change from a traditional deficit -- and that the surplus could continue for some years.

He pointed out that by letting the won appreciate, Korea would encourage additional U.S. imports, and thus help Korea meet another one of its major economic goals -- reducing its $6 billion deficit with Japan.

Bergsten reported that the Korean government recently identified 600 industrial sectors for which they had a great dependence on Japan, and for which they are seeking American or other substitutes.