Users in eZ Publish are actually content objects with an ezuser attribute, so it is straightforward :).
I already made an implementation of user import in my JanrainAuth extension (to connect user from social networks like Facebook). Take a look at the JanrainAuth class, in getUser() method.
You also might want to read the SQLIImport fromString() Appendix to learn how to deal with ezuser datatype. Take also a look at the documentation because there is a set of classes to handle CSV files ;).
