I would like to import CSV or XML files from Tradedoubler and Zanox, so I can use K2 (and K2 store) as a shopping cart.How is this possible ?Kind regards

Will this be a one time import?
Probably a custom import to Joomla article first with an extension to do that, then with fresh k2 import everything, add only to k2 from there. If you are planning to use both systems, then a bridge of some sort will be required.

The CSV import is possible through special component (its usefull if you need to do this very often). I have such written by our team for our needs. I can share it to you.
The only problem is that if you want to fill extra fields and price - there is no such universal solution. However we can patch this component for your needs

Contact me if you are interested.
Skype: maa.wis-art.com