disable user's sieve, but add your own as the admin
zmprov ma firstname.lastname@example.org zimbraFeatureFiltersEnabled FALSE
if the Mom folder didn't exist, the command would create the folder automatically:
zmmailbox -z -m email@example.com afrl "Mom" active all header "from" contains "firstname.lastname@example.org" fileinto "Mom"
(originally from King0770-Notes-Sieve_Rules_By_Proxy)