Virginia Gov. Charles S. Robb, whose life after the governorship is one of the hottest topics of speculation in the state capital, has discussed the possibility of joining an Alexandria law firm when he leaves office early next year.

"We've had some conversations," said William G. Thomas, a close personal advisor to Robb and a partner in the Alexandria firm of Thomas & Fiske.

"He's just started thinking about what he might want to do. It's in the early exploration stage," Thomas said.

George M. Stoddart, Robb's press secretary, said the governor told him yesterday that he is "nowhere near a decision" on future employment. Stoddart said Robb told him that he "is probably a couple of months away from even beginning to narrow his options."

The only public statement made by Robb, who cannot succeed himself as governor, is that he and his wife, Lynda, will move back to their McLean home.

Before he was elected lieutenant governor eight years ago, Robb, son-in-law of former President Lyndon Johnson, worked in the Washington law firm headed by Joseph Califano.

Top officials in Robb's administration speculate that Robb, who many believe is interested in positioning himself to run for national office or the U.S. Senate in 1988, will accept a job that will give him national and international exposure as well as time to raise money and campaign for Democrats in 1988.

Thomas, an influential lobbyist in Richmond who once estimated that he personally tapped his law clients and other friends for up to $200,000 in contributions to Robb's 1981 campaign for governor, said Robb "has had conversations with a lot of people" about his future. As for reports that Robb is considering his firm, Thomas said "there's a little sniff of accuracy."