An administration tax expert warned Congress yesterday not to pass a money-raising package that would reverse "dramatic and important tax reforms," calling instead for $6.1 billion in new fees outlined in President Reagan's $1 trillion budget.

"The administration continues to support the $6.1 billion of additional governmental receipts shown in the president's budget proposal," Dennis Ross, the Treasury Department's tax legislative counsel, told the House Ways and Means Committee.

"The president has made clear his opposition to the budget reconciliation {passed by Congress} and its inclusion of a substantial tax over the next three fiscal years," he added.

"The administration believes it is extremely important that we not undo any of the dramatic and important tax reforms that were accomplished last year," Ross said.

The hearings are the first step by the Ways and Means Committee to meet its obligation under the budget resolution to find a way to raise an extra $64.3 billion for the government over the next three years.

Of that sum, $19.3 billion is needed in fiscal 1988, which begins Oct. 1, $22 billion in fiscal 1989 and $23 billion in fiscal 1990.

According to the plan, the extra tax money is needed to help run government programs while also cutting the federal deficit by about $37 billion.

In a question-and-answer session focusing heavily on the proper definition of a tax, committee members challenged the distinctions between money-raising proposals offered by the administration and those under review by Congress.

"They are revenue increases," Ross said of Reagan's proposals, which include raising fees for people who use federal lands and waterways, requiring employers to pay Social Security taxes on employes' tips, and repealing certain exemptions from gasoline excise taxes.

"We would resist the characterization of them as tax increases," he added, saying tax increases would be a "breach of the pact" reached last year when a law was passed to lower federal income taxes for many Americans.

"I find it difficult to accept the fact that when you make a recommendation, it's called revenue enhancement; if we do it, it's called a tax increase," objected Rep. J.J. Pickle (D-Tex.).

"That appears to be hypocrisy," he said.

Lawmakers have a 291-page list of revenue options, including suggestions to raise taxes on alcohol and tobacco, to assess higher fees on petroleum products and to order new money-raising twists in the federal income tax laws revised last year.

None of the prospects has sparked enthusiasm, and all have been repeatedly denounced by Reagan, who has vowed to veto a tax increase