Hilda H. Mason, an at-large member of the D.C. City Council who advocates low transit fares and completing all 100 miles of the Washington region's proposed subway system, was appointed yesterday to the Metro board.

Mason was chosen by the City Council as an alternate director, with a full right to participate in Metro board and committee deliberations, but with power to vote on the board only if either of the city's two full-fledged directors is absent.

The appointment came at the council's first legislative session of the year. The council did only minor law-making, extending the lives of three emergency bills for another 90 days and logging in 36 new bills introduced by council members.

One of the extended emergency bills maintains a moratorium on the shift of apartment houses to cooperative ownership, while permanent legislation is being considered. A similar moratorium is in effect on condominium conversions.

Mason, a Statehood Party member, said she plans an active role at Metro. She replaces council member Willie J. Hardy (D-Ward 7) who rarely attended Metro meetings.

The council reappointed Jerry A. Moore Jr., (R-At Large) as one of the city's two full directors, putting him in line for the Metro chairmanship this year. No decision was made on the other directorship previously held by former Mayor Walter E. Washington and the alternate seat held at D.C. transportation director Douglas N. Schneider Jr.

Council Chairman Arrington L. Dixon said he wanted to talk to Mayor Marion Barry before recommending those appointments. Dixon said he expected Schneider to continue to serve until a decision is made.

The council also named John L. Ray, who was sworn into office as an interim Democratic replacement in Barry's former at-large council seat, to three committees -- human resources, transportation and judiciary.

Dixon said he had designated Betty Ann Kane (D-At Large) to serve on the board of directors of the Metro-politan Washington Council of Governments and Nadine P. Winter (D-Ward 6) to continue on the board of the Pennsylvania Avenue Development Corp.