U.S. Immigration and Naturalization Service Commissioner Gene McNary is seeking to increase Border Patrol staffing in San Diego by about 18%, to 1,000 agents, perhaps by the end of this year, his chief spokesman said. The hirings would make 1990 the year in which the San Diego branch of the agency received its largest staff increase ever. The steep increase would follow a yearlong, 1989 hiring freeze that left the patrol severely undermanned, officials said. The expansions are targeted at understaffed San Diego--the busiest illicit crossing along the almost 2,000-mile U.S.-Mexico border--and do not represent increases nationwide, said Verne Jervis, spokesman for INS Commissioner McNary. Jervis and McNary visited the San Diego-Tijuana border area this week.