General Motors Corp. announced today that it is expanding a program providing cash incentives to dealers to include a luxury Cadillac model, the front-wheel-drive intermediate A cars and light-duty pickup trucks.

GM's announcement came as industry reports today showed that sales of U.S.-made cars plummeted 26.4 percent in the first 10 days of August from 1981 levels.

The automakers said they sold 112,839 cars in the Aug. 1-10 period compared with 153,383 last year. The daily selling rate of 14,104 was the worst since 13,458 in Aug. 1-10, 1958. If cars continue to sell at the Aug. 1-10 rate, automakers would sell 5.6 million compared with an 8.1 million annual rate last August.

One analyst said the poor figures mean automakers have no hope of matching the strong sales of August 1981, when 601,741 cars were sold with aggressive incentive programs.

GM posted the largest decline of the Big Three automakers, with sales down 29.1 percent from last year. A GM spokesman pointed out that the first 10 days of August 1981 marked the beginning of an offer of 13.8 percent financing on new car loans.

The No. 1 automaker sold 71,473 cars versus 100,811 last year. So far this year, GM has sold 2.1 million cars, down 11.8 percent from 2.4 million to date in 1981.

In an effort to get rid of big backlogs of unsold cars, GM said the new incentives range from $1,275 on the Cadillac Seville to $350 on Chevrolet S-10 trucks and GMC S-15 pickups. Except for the trucks, the money will be paid for each 1982 vehicle sold and will not be based on sales quotas as is often the case on dealer incentives, GM said.

"We hope a lot of it the money is passed on to customers," spokesman Harold Jackson said, but dealers can use the incentive money as they wish, perhaps for sales personnel bonuses or advertising.

GM said it does not plan to pay any more direct cash rebates to consumers in this model year, but has made no decisions on 1983 models.

Ford Motor Co. is offering dealer incentives and a warranty program, Chrysler Corp. has a warranty program, and American Motors Corp. and Volkswagen of America Inc. have cut prices.

Meanwhile, Ford reported a 21.7 percent drop in sales for the first 10 days of the month, a decline that was expected by the automaker. Ford sold 23,530 cars compared with 30,036 autos last year.

Chrysler said its sales dropped 20 percent to 13,676 from 17,009 in the first 10 days last year.

American Motors Corp. sold an estimated 1,900 cars in the first 10 days, down 30 percent from 2,711 last year.

Volkswagen of America sold 2,260 cars in the first 10 days of August, a 19.8 percent drop from 2,816 last year.