To add an interface to a zone by editing the ifcfg-em1 configuration file, for example to add em1 to the work zone, as root use an editor to add the following line to ifcfg-em1:

ZONE=work

Note that if you omit the ZONE option, or use ZONE=, or ZONE='', then the default zone will be used.

NetworkManager will automatically reconnect and the zone will be set accordingly.