Poland's financial difficulties could add more than half a billion dollars to the U.S. budget deficit in the current fiscal year, according to Reagan administration officials who are battling to narrow the gap between spending and revenues.

The half-billion-dollar figure assumes that Poland's hard-pressed central bank will be able to pay only a small part of the $658 million in principal and interest owed to two main U.S. government lending institutions, the Commodity Credit Corp. and the Export-Import Bank, in fiscal 1982. Many financial experts say they seriously doubt whether Poland will be able to come up with all it owes.

The new Polish military regime has already asked western governments to refinance loans due in 1982, meaning to postpone repayment, a step that would reduce U.S. budgetary receipts and increase federal outlays to private American banks whose loans are guaranteed by the government. The possible shortfall in receipts from Poland comes at a time when the administration is already heavily cutting the budget to keep down the likely deficit. The administration is cutting a number of domestic programs half a billion dollars and more, and paying a high political price for that.

Since the start of the fiscal year in October, small amounts of money have trickled in from Warsaw, but Poland remains behind in its payments to the two lending institutions and officials acknowledge that it is anybody's guess how much more will be paid in the months ahead.

Before the Polish military imposed martial law, the U.S. government agreed to let Poland postpone paying all of the principal and 90 percent of the interest owed to U.S. government lenders in the 1981 calendar year.

The bulk of the $375 million refinanced in this way was charged against the 1981 budget. But officials of the Office of Management and Budget and the Commodity Credit Corp. say Poland will have larger obligations coming due in 1982 because of substantial private bank loans guaranteed by the government over the last 24 months for the purchase of U.S. agricultural commodities.

The delay in repayment of the Polish debt affects the federal budget two ways.

First, the delay in repayment of U.S. government credits reduces the amount of revenue coming in. In fiscal 1982, Poland is supposed to pay back $310 million to the Commodity Credit Corp., the Agriculture Department's bank, and another $40 million to the Export-Import Bank. If that money is not paid it will have to be made up by government borrowing.

Second, the government has to make outlays to the private banks to cover loans it has guaranteed. In fiscal 1982, $308 million in loans guaranteed by the Commodity Credit Corp. is coming due.

So far, according to CCC Treasurer Robbie A. Brammer, no claims have been made by the banks, but the CCC is obligated to cover them if they come in as expected.

The CCC will get this money back starting in 1986 under the terms of the refinancing agreement, but in the meantime the loss of receipts and the outlays to the banks show up in federal budgets whose deficits have become a major political issue in the Reagan administration's economic program.

The uncertainty about Polish repayments is coming at a time when the Commodity Credit Corp. is already running short of funds to run the domestic farm programs that are the core of its operations.

Commodity Credit functions like a bank, lending money to farmers and foreign governments, reselling commodities it has obtained under price support programs and recirculating money collected from old loans. When Commodity Credit needs more money it borrows it from the Treasury and pays back the Treasury out of its own revenues.

However, heavy payments recently to cotton and wheat farmers have pushed the agency up near the limit of the $20 billion borrowing ceiling set by Congress in 1979.

This has caused a liquidity problem for the agency, which has now used up $18.2 billion of the total $20 billion. Earlier this month it deferred a $1.6 billion interest payment to the Treasury because of the tight funds situation.

Officials of the government bank acknowledge that nonpayment by Poland of the money owed could affect operations unless Congress raises the borrowing limit.