I resolved it in a creative way.
The first issue were that k2 content can not be migrated to joomla for free if you want to move the content images too.
The first step was generate an RSS XML with all the posts and try to import them into my wordpress site.
To import them in wordpress I used WP All Import
This plugin moved the content from joomla to wordpress in some minutes INCLUDING THE IMAGES!
Nice!
But the images were linked to the joomla domain.
Our next step is retrieving each post image from joomla to wordpress and fix the links.
For this task I used Auto Upload Images
Its process take about 10 minutes or more, but with amazing results.
The last step was to use the image of each post as excerpt.
Unfortunately I did not find plugins to do it, so it was a manual process.
Hope to be helpfull!