President Reagan has conditionally agreed to participate in a summit conference with about 20 leaders later this year in Mexico to discuss North-South economic issues, according to diplomats trying to arrange the meeting.

But any U.S. acceptance of the summit conference apparently does not imply any shift in the Reagan administration's initial skepticism regarding multilateral development efforts and its preference for bilateral aid and trade as a path to global economic growth.

Still, the U.S. show of interest in the meeting will gratify France, West Germany and other European countries that want Reagan to avoid treating Third World issues entirely in terms of the East-West conflict, the diplomats said.

A group of 11 foreign ministers, who arrived here today for a two-day preparatory meeting starting Friday, can now set in motion the final preparations for the summit meeting.

The biggest remaining hurdle for the meeting, originally planned for June, is finding a later date. One of Reagan's conditions reportedly is a postponement for several months.

The official U.S. explanation is that the new administration needs time to prepare. Diplomats said, however, that Reagan wants the Western economic conference in July in Ottawa -- where the United States is to meet with six other major industrial nations -- to precede any discussion of global economic issues in a broader forum.

A key condition for U.S. participation in the Mexico summit meeting reportedly is that it be a discussion among heads of state, not a negotiating session. Reagan spoke by phone March 6 with President Jose Lopez Portillo of Mexico, diplomats said.