The House Appropriations Committee yesterday approved an urgent supplemental spending bill that would provide about $1 billion in new money for overseas embassy security, air traffic control, congressional mailing and a raft of other programs during this fiscal year.

Although most of the new money was appropriated to cover unanticipated program costs and provide disaster relief to flood and hurricane victims, it included $454 million to begin a long-term antiterrorist security program at U.S. diplomatic installations.

In a major policy step, the committee approved a motion by Rep. David R. Obey (D-Wis.) requiring that the antiterrorist money be taken from already appropriated Defense Department and foreign aid accounts. Otherwise, Obey argued, the committee would be forced to violate Gramm-Rudman-Hollings deficit-reduction limits.

The spending measure also included language by Chairman Jamie L. Whitten (D-Miss.) that could limit enrollment in the new federal soil conservation reserve, which will farmers to retire highly erodible land from production.

Rep. Silvio O. Conte (R-Mass.) said he will propose changes in Whitten's conservation proposal when the bill goes to the House floor after the Easter recess. The administration opposes the changes authored by Whitten.

Although Whitten urged the committee to "hold ourselves in check," amendments adopted yesterday apparently put the bill over the $901 million limit on additional spending. Highlights included:

*A $340 million appropriation for the Internal Revenue Service for improvements in tax-return handling procedures and to speed up payment of income tax refunds.

*$150 million for the Environmental Protection Agency "Superfund" toxic-waste cleanup program, which ran out of funds after Congress and the administration stalemated over a reauthorizing bill.

*A $42.2 million allocation to help pay a congressional mailing bill that Rep. Vic Fazio (D-Calif.) said could hit $166 million this year.

The House passed and sent to President Reagan a compromise bill to provide another $5 billion this year to the Commodity Credit Corp. for crop price-support programs.