I want to send custom protocol between Xmpp clients, the server is Openfire. For example, If client A want to establish a P2P call with client B, then A can send something like < iq > < call />< /iq > to B. As far as I know, if A want to send B the custom < iq />, then the server needs to create a new plugin to handle this < iq /> packet. Is there anyway to implement this idea without the server change?
This depends on the server, but usually if you have an IQ stanza that is correctly addressed (from, id and complete FullJID of the destination) then the server should forward that stanza to the user just fine. The most important bit - it has to be FullJID (with resource) of your contact.
(copied from comment after indication that this worked for OP)