Senate Finance Committee members agreed yesterday to make the country's employers and workers buy an added $6.4 billion a year worth of health insurance to cover all medical and hospital bills over $3,500 a year.

The step is the most important so far in a determined attempt of chairman Russell Long (D-La.) to write the first new national health insurance legislation since Medicare.

The Carter administration's proposal, which would cost $7.8 billion, would cover every employe, or employe and family whose bills run over $2,500 in any year.

President Carter said his plan would cost employers $6.1 billion a year and employes another $1.7 billion, including the cost of some maternal and infant care benefits and subsidies for some employers.

Finance Committee calculations - or far not including these added elements - say the president's $2,500-and-over catastrophic benefit would cost employers an added $5.4 billion a year and employes $1.8 billion for a $7.2 billion total. But the committee's $3,500-and-over limit, it is said, would cost employers just $4.8 billion and employes $1.6 billion (in both cases assuming employers paid three-fourths of the premiums).

The administration would also provide coverage for the self-employed and others who cannot buy adequate coverage today at affordable prices. The Finance Committee agreed to guarantee "access to a qualified insurance pool" to the self-employed, but not to make them buy coverage.

What the Finance Committee has been conducting is a day-by-day discussion of many possible health insurance details, with Long pushing hard for agreements, but always agreements limited to "what we can afford."

What Long and the committee think the nation can afford is sure to differ sharply from the president's overall health insurance proposal, which carries a $24.3 billion price tag, in federal and private contributions. The president proposed billions in new aid for the poor, aged and others, as well as the catastrophic insurance benefits.

Long has said he would support a $20 billion package rather than see no health bill, but "all" he will seek is perhaps a $10 billion to $12 billion package.

The poor, Long said yesterday, may have to be among those on the waiting list.

"We want to try to cover everybody", starting as early as next year," Long said. "The poor will be better protected next year than now. The following year we'll do better by them, and by 1983, the year the administration proposal would start [taking effect], they'd be much better off.

"We have to start with the money we can find. You don't try to help everybody."