A majority of the Senate either favors an end to charging food stamp recipients for the grocery-buying coupons or is leaning that way, an officer of the Women's Lobby, Inc., said yesterday.

The so-called "free food stamps idea" was first proposed 18 months ago by Sens. Bob Dole (R-Kan.) and George McGovern (D-S.C.), but has never been tested by a Senate vote.

The Carter administration is considering proposing such a change.

Government experts estimate that it would add $400 million to $600 million a year to the budget by attracting to the program many of the roughly 6 million eligible families who don't now receive food stamps.

Pam McEwan, director of food-stamp reform for the Women's Lobby, told the Senate Agriculture Committee that a survey of 99 senators by her group found 31 senators favoring elimination of the purchase requirement for the stamps and an additional 26 favorable to it but undecided. The two groups would constitute a majority if all the senators voted in favor of the requirement.

She said that 11 were opposed, four were not favorable to it but undecided and 27 were undecided without leaning either way. Sen. John C. Stennis (D-Miss.) refused to participate in the survey.

The food-stamp program now serves about 5.6 million families at an annual federal cost of about $5.4 billion. It expires Sept. 30, along with most other major agriculture department programs. The committee is taking testimony on proposed replacements.

Under the present system, eligible families buy stamps. The average food-stamp family, consisting of three persons, pays $58 for food stamps which will purchase $130 worth of groceries.

If the charge were eliminated, the family would pay nothing and receive $72 in food stamps.

Opponents say the proposal would destroy the purpose of the program - to provide a nutritional supplement for the poor. The present system forces food-stamp families to spend a given amount each month on food, they argue.

But Childrens Foundations officials, New Jersey Human Services Commissioner Ann Klein and others testified that the cash charge is the single greatest barrier to the program for those who need it most.