Pacific Southwest Airlines obtained the modifications from its 45-member Southwest Crew Controllers Assn. employee group. The San Diego-based airline's $400 million acquisition by USAir Group is contingent upon PSA obtaining modifications of labor contracts that give organized employees added rights in the event of a takeover or merger. PSA has negotiated similar modifications with its pilots' and flight dispatchers' unions and will resume negotiations this morning with the Teamsters Union. In San Francisco, meanwhile, a federal judge ruled against a Teamster request that PSA be forced into binding arbitration in the contract dispute. The Teamsters plan to appeal.