The Defense Department is working on legislation that would authorize drafting of up to 100,000 Army veterans as replacements for Troops felled in the first days of a war.

Pentagon civilian executives said yesterday that analysts have concluded that veterans are the best source for skilled combat troops that could be put into action in a hurry.

Army leaders, however, oppose the idea, contending it would be "inequitable" to force some men to serve twice without drafting those who had not served at all.

"The issue is urgency against inequity," said one Pentagon executive yesterday.

The idea is to conscript trained-for-combat veterans but exempt those who have been in battle. Under current plans, this would exempt most Vietnam veterans. Also, the Pentagon wants to limit such a draft to men 30 or under. This also would reduce the vulnerability of Vietnam veterans to such conscription.

If the plan under consideration became a reality, the president at the outset of a national emergency would issue an executive order directing veterans and others to register at the voting station nearest their homes. This could be done without a new law because registration remains on the books although the registration requirement was suspended by executive order in 1975.

Once the veterans had registered, the Army would sift through the records of those 30 and younger who were trained as artillerymen, infantrymen, tank drivers, combat engineers and medics. Army leaders agree that these skills would be greatly needed in the first four months of a war.

However, Congress would have to pass legislation to give the president authority to draft veterans or anybody else. The Pentagon has not yet decided whether to send such legislation to Congress this year, assuming it is approved within the executive branch, or to hold back such proposals until a national emergency occurs.

The basic problem is that the Army no longer has the veterans it would need in the early stages of war in the Individual Ready Reserve where they could be activated as replacements.

Gen. Bernard W. Rogers, who served his final day yesterday as Army chief of staff, acknowledged the problem at his farewell news conference.

"Even if we had registration today," Rogers said, "we would run out of infantrymen, tankers, artillerymen and medics before that 120-day period" to train draftees is up. "That is the major challenge that I see in the volunteer environment as far as the Army is concerned and as far as the country is concerned," he said.

"There is a school of thought," he continued, "that says we should have an emergency reserve" consisting of veterans who would be drafted. "To me that does not seem equitable that we would draft a veteran to serve when there are so many others who have not yet served their country."

Army Secretary Clifford Alexander said in an interview yesterday that he also opposes drafting veterans because it would be "inequitable."

Alexander said that more equitable solutions that the Army itself is examining would include calling up retirees with special skills and extending the total obligations of men who come into the service in the future to try to correct reserve shortages. He said the Army is also working on incentives to draw more men into the reserves.

Chairman Sam Nunn (D-Ga.) of the Senate Armed Services manpower subcommittee has been arguing that resuming registration of young men would be a fairer solution than resorting to such Pentagon plans as drafting veterans. He is sponsoring a bill to require registration.