Currently, admins can set multiple forwarding addresses for an account by setting zimbraMailForwardingAddress. (Or see admin console > individual account > forwarding tab > forwarding addresses hidden from the user.)
The end user can only set one value in their options/preferences (zimbraPrefMailForwardingAddress) which is a single valued in the LDAP schema.
Making it multi-value really calls for having an admin settable limit on how many you are allowed. Be sure to vote on:
Bug 11299 - Allow user to set multiple forwarding addresses, need admin defined limit