gnus-category-mode-hookgnus-category-line-formatgnus-category-mode-line-formatgnus-agent-short-articlegnus-agent-long-articlegnus-agent-low-scoregnus-agent-high-scoregnus-agent-expire-daysgnus-agent-enable-expirationENABLE which
means that you'll have to disable expiration when desired. On
the other hand, you could set this to DISABLE. In
that case, you would then have to enable expiration in selected
groups.