James Chikerema, a veteran black nationalist allowed by the government to return from 13 years of self-exile, said today that he supports two of Prime Minister Ian Smith's key conditions for majority rule.

These are retaining the existing armed forces under the command of white officers and allowing the white minority to have a parliamentary "blocking mechanism" under black-majority rule.

Chikerema's surprise statement came one day after he returned from exile, most of it spent in Zambia, Chikerema would not say how his return was negotiated, but said he had made no deals in exchange for permission to come home.

Chikereman, 52, who had been regarded as a militant among the many black factions fighting for majority rule, is the first black leader to come out publicly in favor of any major part of Smith's peace plan. Britain and the United States are trying to end guerrilla warfare in Rhodesia by negotiating a way to transfer power from the territory's 270,000 whites to its 4 million blacks.

Just how signifcant Chikerema's move is remains to be seen, since he has not been among the forefront of Rhodesian black leaders for years. There were reports from London that Chikerema was believed to be in the pay of the Smith government and that his return was orchestrated for just such an announcement.

Smith has been trying for two months to negotiate what he calls an "internal" settlement, meaning reaching a deal with blacks within the country who are regared as less militant than the exiled leaders carrying on the guerrilla war from outside Rhodesia's borders. By reaching such a settlement, Smith hopes to be able to bypass the proposed Anglo-American plan for majority rule, which would be less favorable to whites.

Previously known for his militancy and widely regarded as a founder of black nationalism in Rhodesia, Chikerema is first vice president of Bishop Abel Muzorewa's United African National Council.

Muzorewa and his rival, the Rev. Ndabaningi Sihole, are the two nationalalists Smith has indicated he would like to involve in an "internal" settlement if the curremt Anglo-American initiative fails.

Chikerema said he had returned to help settle the countty's problems by peaceful negotiation. He said he thinks that whites "should be reassured by affording effective guarantees of individual and group security in our future consitution such as a blocking mechanism in Parliament."

He added: "The country's security forces should remain intact because that will be the best reassurance to anyone genuinely concerned fot the future and peace and welfare of our country, as opposed to those concerned only with their own particular power irrespective of the cost to the state."

Chikerema said he has "one qualification" to retaining the armed forces as they are, but did not elaborate.

He called his views on Smith's plan personal ones and "at this stage" not the policy of the United African National Council.

The nationalalist leader attacked the "self-appointed referees" in the Rhodesian issue. He said later he was referring in particular to the president of the five African "front-line" states, which support the guerrilla warfare against Smith.

Chikerema said he still supports the nationalist guerrillas.

"The struggle is being fought for a geniune cause, and until that cause is settled the armed struggle will go on," he said. "Let's face it, 99 per cent ofthe people of this country are supporting the struggle."