In a direct challenge to congressional and financial industry critics who oppose conversions of depositor-owned thrift institutions into stock corporations, the Federal Home Loan Bank Board approved additional applications yesterday.

Two federally-chartered savings and loan associations-the $208 million First Federal S&L of Raleigh and $230 million North Carolina Federal S&L in Albermarle Country, N.C.-won Bank Board blessings for their conversion plans.

The New York-based Council of Mutual Savings Institutions, a consistent opponent of the conversion process, immediately denounced the federal regulatory agency for "taking an illegal action not approved by Congress and one which is in the private, rather than the public interest."

Legislation passed in 1974 permitted the Bank Board to approve some conversions of mutually-owned thrift institutions into stock corporations on an experimental basis. At issue is whether the regulatory agency still has authority to approve conversions, since a two-year period covered by the law has expired.

Bank Board Chairman Robert H. McKinney, who recently announced plans to leave the agency sometime during the summer, asserted yesterday that lawyers for the FHLBB "are confident of the Bank Board's authority and responsibility under existing law to approve federal mutual to federal stock conversions,"

McKinney called yesterday's action a "significant step forward in this agency's progress toward providing new economic opportunities for our member savings associations."

Among the principal arguments advanced in favor of conversions to stock corporations is providing a method for S&Ls to raise more capital in an era of growing financial institution competition for similar business services. Many state-chartered S&Ls have been permitted to become stock corporations and raise additional capital - posing a threat to federal S&Ls not allowed the same access.

Theoretically, S&Ls have been owned by their depositors and such customers are given rights to buy stock in conversion plans. Although depositors are owners of mutual S&Ls - including most in the D.C. area - they seldom attend annual meetings or have much of a say in management.

According to Kenneth Virch, executive director of the Council of Mutual Savings Institutions, most depositors of S&Ls that have been coverted lost what interest they had in the associations, even if it was seldom exercised.

To date, he said yesterday, less than 4 percent of depositors purchased stock while the 96 percent who did not buy shares lost their voting rights without compensation. "Conversions are a rip-off of the mutual members' rights," he charged.

The legality of S&L conversions is being challenged in federal court here and the General Accounting Office is preparing a study of conversions at the request of Senate Banking Committee Chairman William Proxmire D-Wis.), who has contended that further FHLBB conversion approvals are illegal.

After criticism of earlier conversions the Bank Board issued new regulations which were designed to prevent abuses by insiders of the S&Ls. Under new rules, purchases of stock by officers and directors is limited to 5 percent for an individual and 25 percent for all such persons. Any shares left after an initial offering to the general public whereas in the past, officers and directors often purchased large blocs of the shares.