Hi Luca!
Actually we upgraded our system long time ago! You can now keep your previous data without any problems! You just need to uncheck "Clean migration" option and all of your previous data will be there!
However, I am not sure that I understand you completely about "dedicated category"... When you run migration, all of your WP categories will be migrated. If you have some blog post from WordPress with more then one category, CMigrator will add that post only to one of them. If you want to migrate all of your posts and pages from WordPress to one, single category, just uncheck "Migrate Categories" option and all data will be placed in 2 categories, "Uncategorised pages" and "Uncategorised posts". You can than rename those categories or move all data to one single category from K2 component later.
If you want, we can go through migration together. I am always here to help!
Best Regards,
Petar