Indeed there is no plugin for that, to have it that user friendly such in User export plugin. But you could simple copy the "ofmucroom" table from you old system to the new one.
Or you can use the REST API plugin (Ignite Realtime: Openfire Plugins ). You request all chat rooms from your old system by call: REST API Plugin Readme
and add the rooms to the new system by: REST API Plugin Readme .