I'm moving to Krakow an I have a small collection of firearms inherited from my father. Is there some possibility of importing them to use them in fire ranges or eventually go hunting?
You're more likely to get a useful answer on the International Practical Shooting Confederation forum: ipsc.invisionzone.com/
You'll have to create an account and wait for it to be approved. Ask in the section called "The Road Warrior", which is for advice about travelling with firearms.
Also, it would be a good idea to join the IPSC. It would make demonstrating a need to import the weapons a lot easier. If you don't have a compelling reason to import the guns, it is likely that your request will be denied. Being a registered competitive shooter is a compelling reason. Being a hobbyist collector is most likely not.
The other organization that is worth looking into is the ISSF (International Shooting Sport Federation). They don't seem to have a forum, though. Just to cut you off at the pass if you're heading in that direction, no, the NRA is not a legitimate organization for this purpose.