Another missing black child from an impoverished neighborhood was added to Atlanta's list of missing and murdered children today, swelling the caseload under investigation by the special police task force to 22 -- 20 children murdered and, now, two missing.

While Public Safety Commissioner Lee Patrick Brown was telling reporters here that the case of 16-year-old Joseph Bell seemed to be linked to the child murders that have been plagued this city, President Reagan announced that he has approved the city's request for $1.5 million to help pay for the investigation.

Until today, Bell's case was being investigated by the police missing persons bureau, which was unable to substantiate reports that he had been seen since he left his part-time job sweeping up at Cap'n Pegs Seafood Carryout to play basketball March 2. The next day, his brother reported him missing. Also missing is Darron Glass, 10, last seen Sept. 14.

A slim, street-wise youngster, 5 feet 5 and 100 pounds, "Jo-Jo" Bell had a fine arc on his jump shot and he dreamed of becoming a basketball star, he told friends. He grew up fast, like so many of the other missing and murdered children, hustling for a buck in the shadow of seedy downtown housing projects as one of seven children born into a broken home.

For the last three years, he lived with his grandmother while his mother served a prison term for killing her husband. Recently released from jail, the mother, Doris Bell, said, "Deep down in my heart, I don't feel like Jo-Jo's dead. I won't let it touch me that he's dead. But I'm hurting more now than I ever hurt in my life."

In Washington, far away from the Bell's gray clapboard house, Reagan announced approval of the funds at a White House new conference, deploring the perplexing murders of black children -- 18 boys and two girls -- as "one of the most tragic situations that has ever confronted an American community." He also said Vice President Bush would fly to Atlanta Saturday to confer with city officials in a show of administration support.

The coveted $1.5 million is earmarked for the extraordinary costs of police overtime, running at $150,000 to $170,000 a month, according to city officials. Reagan said the new grant would augment $997,000 in federal aid dispatched to Atlanta March 5 for youth protection and mental health programs -- federal help that was criticized by some local black leaders because it couldn't be used to track down the killer or killers.

"This administration is doing and will continue to do what we can to bring an end to this tragedy," Reagan said, as he prepared to travel to New York tonight.

Asked if the administration would have responded sooner had the victims been white -- a perception shared by many in the black community -- Reagan said: "We moved as quickly as we could. This administration is totally color-blind."

Mayor Jackson, who listened to the news conference, emerged from his office to tell reporters, "In the midst of continuing sorrow, today is a day for good news for Atlanta." He called the president's words "sensitive, caring and, clearly, with Vice President Bush coming as President Reagan's personal emissary -- even additional evidence of the continuing concern the Reagan administration has shown for Atlanta's awful tragedy."

But public criticism about police handling of the investigations continued today when a former homicide detective, W. K. Perry, accused the special task force of failing to respond to a frantic phone call from one of the murdered children, Patrick Baltazar, before he disappeared.

Perry, who is conducting his own private investigation into the killings, played a tape of his interview with a 10-year-old playmate who is believed to be the last person to see Baltazar alive.

The unidentified youngster said he was playing with Baltazar near the south Atlanta public housing project where he lived when a "big man" ordered them to "come here, you two boys."

"Patrick started getting into the car and I grabbed him from behind and said, 'You don't know who that man is.' He [the man] said, 'I'll be back,'" and drove off, the youngster told Perry, and the boys ran to a pay telephone. Baltazar dialed the task force. He told them he was being chased by a man. He gave his name and address. He was told that a patrol car was on the way, according to his friend, and the boys waited at the booth for a short time, grew fearful and split up.

Investigators theorize that Baltazar doubled back to take down the car's license plate in the hope of earning the $100,000 reward offered for information leading to the killer's arrest. He was never seen again.

That was Feb. 6, but the special task force didn't contact the playmate for questioning until this week -- one month after Baltazar's body was found in a gully beside a DeKalb County office complex.

"It's surprising they would let this go on this long," said Perry. "They're only a month and a half late."

Public Safety Commissioner Brown said he was aware of the charge, but had no comment.