The federal government's expanding appetite for "backdoor" borrowing by the use of guaranteed loans needs to be controlled, administration "officials agreed yesterday although they warned it will not be easy.

An estimated $422 billion of federally guaranteed loans will be outstanding at the end of the 1981 fiscal year. These are issued by private lenders for a wide range of programs, from mortgage assistance and "small business loans to support for synthetic energy projects and the Chrysler Corp. bailout -- assuming that receives final approval.

These credit demands have been rising fast, noted Assistant Treasury Secretary Roger C. Altman, testifying before a special Senate subcommittee on federal credit. The annual increase has grown from $16.3 billion in fiscal 1976 to an estimated $50.3 billion this fiscal year.

Sen. Howard M. Metzenbaum (D-Ohio), chairman of the select subcommittee, contends the government is losing control over these credit programs.

"They have come to be used as a backdoor method of funding which permits billions of dollars to be spent outside the scrutiny of the budget process," he said. And the more control Congress exerts over direct spending through the budget process, the more popular such "backdoor" methods of financing through government-guaranteed loans become, he complained.

Metzehbaum charged that some federal agencies are not scrutinizing loans and loan guarantees carefully enough to identify potential losses through default. The losses become federal spending loans, he said, "yet we have no control over that."

Richard Kezer, a senior vice president of Citibank, minimized the effect of federally backed borrowing on the credit markets, saying it has had little impact, on inflation rates and has not crowded out private borrowing.

Sherman Maisel, a financing expert with the University of California at Berkeley agreed with Altman that the starting point for better control was with the federal agencies that supervise the government credit programs and the congressional committees that oversee the agencies -- rather than the Treasury Department, which lacks the information and authority to regulate the borrowing.