Update the Sieve script in gnus-sieve-file, by replacing the region
between gnus-sieve-region-start and gnus-sieve-region-end with
(gnus-sieve-script gnus-sieve-select-method gnus-sieve-crosspost), then
execute gnus-sieve-update-shell-command.
See the documentation for these variables and functions for details.
(fn)