wud'nt q/say 'easy..' but export-joomla-kunena/reimport-elgg can be done -- just need to figure out the data structures @ source and target and offload, reformat & reload.(btw - kunena looks knifa intrsting. maybe look into integrating as a plugin into elgg..

yes i checked it last night and it can be done from the database but it want be easy..
i was hoping for a solution. i found something checking google but its for an old version of kunena and i dont thing that it will work. i will give it a try and inform if it can be made.
if you decide to make kunena plugin i can help on testing and maybe in translation to Greek.

You can have two approaches - either integrate kunena into Elgg (harder), or import data from your forum to some existing forum script. In second scenario, I would advice you to do import in object model. Doing import on database layer (especially in case you don't have large experience with Elgg) can result in some serious problems in Elgg data structure later.

There are converters available for Kunena to other forum softwares like phpbb, smf and punbb etc. Then you can simply create a bridge software between Elgg and your forum software. What we have done last time is converted kunena to smf forum and from smf we converted it to punbb/fluxbb (kunena->smf->fluxbb). You can stop at the first conversion itself and create a bridge for Elgg and Smf. But we needed to convert it to fluxbb again due to specific requirements and we used elgg-fluxbb bridge for it.