Hello Eero, we do not have a import-export solution for this at the moment.
More than 4000, that's a lot of items.
It should be possible, it depends on how many for example extra fields are made in K2, image galleries etc.
At the settings screen in JSW CRM you can make a export of all the items that are present in JSW CRM, categories, item, menubuttons etc. to a csv zipfile.
This is there to easily move a JSW CRM installation from one website to another if you want.
The items in JSW CRM are stored in the contacts table. So you have a idea how the items database is set up.
Perhaps there is a solution for your problem.
It is always possible to copy paste the content of a k2 item on code level to a JSW CRM item.
We could look in to that if you want?