With respect to the national stockpile, the role of los alamos national labs is

Increase existing stock of weapons.

Explanation:

Los Alamos National Laboratory is a federal laboratory owned by the United States Department of Energy (DOE), managed by the University of California, Los Alamos, New Mexico. The laboratory is one of the largest multidisciplinary scientific institutions: with approximately 10,400 University of California employees and approximately 2,800 employees, it is the largest employer in northern New Mexico.

Approximately 1/3 of the laboratory's technical staff is composed of physicists, 1/4 are engineers, and 1/6 are chemical and material specialists; the remaining staff work in Mathematics and Computational Science, Biology, Geosciences, and other disciplines. Professional scientists and students also travel to Los Alamos in the category of visitors, with the intention of participating in scientific projects. The various teams of the Laboratory collaborate with universities and industries in fundamental and applied research, in the sense of developing nuclear warheads and other weapons.