A Washington man whose life sentence for murder was commuted by President Ford after he started a drama group at Lorton Reformatory pleaded guilty yesterday to selling more than a half-pound of cocaine to an undercover drug agent for $10,000 and to taking more than $45,000 from a children's psychiatric center.

Rhozier T. (Roach) Brown Jr., 43, pleaded guilty in federal court here to the two charges, which carry a maximum of 30 years in prison and a fine of $1.25 million. In return, prosecutors agreed not to charge Brown for his alleged participation in a similar theft of a $510,751 check from a Houston firm.

Brown also faces seven years of "back-up" time, the period remaining of a special 19-year parole term imposed by Ford in 1976 when he commuted Brown's sentence, citing Brown's "extraordinary contributions" to the Washington community. While at Lorton, Brown formed a dramatic troupe, "Inner Voices," which was praised for its vivid dramatization of social problems and as a means of rehabilitating inmates.

At the time of the commutation, Brown had served 10 years after having been convicted with two other men of a slaying that occurred during a 1964 robbery in Southeast.

During yesterday's 30-minute hearing, Brown spoke only briefly, telling Gesell, "Yes, I did," when the judge asked if he had sold the cocaine to Drug Enforcement Administration Agent Emile Manara on April 3.

Manara testified at an earlier hearing that the purchase from Brown was to be part of a three-kilogram deal, for which the two men had negotiated for about two months. Assistant U.S. Attorney Roger Adelman said the cocaine Manara received from Brown tested as 63.1 percent pure. Cocaine sold on the street is usually about 20 percent pure.

Assistant U.S. Attorney Theodore Shmanda told U.S. District Judge Gerhard A. Gesell that Brown set up a fraudulent bank account into which more than $45,000 was siphoned from an investment fund of the Hillcrest Children's Center.

Hillcrest is a successor organization to the Washington City Orphan Asylum, the oldest charitable institution in the city, of which Dolley Madison was the first director.

Shmanda said in court that Brown set up an account at the Riggs National Bank in the name of Reginald Mathis and then, pretending to be Hillcrest's director, persuaded a local investment group to transfer funds from a Hillcrest fund into the Mathis account.

In the Houston check incident, more than $400,000 was wired to a Bahamian bank account, sources said. Most of the money was recovered and two other men, including Brown's half brother, have been convicted in the case, according to court records.

Jeffrey Jacobovitz, Brown's attorney on the drug charge, asked Gesell to consider sending Brown to a private drug treatment center in New York state.

Gesell, who did not set a sentencing date, advised Brown to tell his probation officer "anything you want me to be aware of in your history, how you got into this mess" so he could determine how to sentence Brown.