W.C. and A.N. Miller Realtors has become affiliated with Cross Country Relocation Service, a worldwide referral network of independent Realtors based in Atlanta . . . For a number of local firms, 1982 is turning out to have been a pretty good year after all: Ryan Homes notes that it sold 2,800 homes in the Washington area, more than 18 percent of all the new homes sold here. Scarborough Corp., a division of Weyerhaeuser Real Estate Corp., reports more than $16 million in new homes for the year, and is projecting an increase to $21 million this year. Lake Ridge Development Corp. reports the best year ever at Lake Ridge, its 3,500-acre planned community along Lake Occoquan at the Prince William-Fairfax line. In 1982, Lake Ridge sold 494 homes for a total volume of $50 million and a 24 percent increase over 1981. Porten Sullivan Corp., a major builder of multifamily housing in the area, has announced a sales volume in excess of $30 million, representing 320 net sales. And the year was very good indeed to Chris Stengel, who just sold the 245-room Hospitality House Motor Inn in Crystal City for $12.5 million. He bought it just over a year ago for $8.7 million . . . The Northern Virginia Apartment Association is holding a seminar on cable TV in apartments next Saturday. It's free to those in the industry but reservations are required: 671-6777.