Hello,
Well, it is not recommended to do export / import between different version of Kentico
CMS (5.0 vs 5.5R2). You should upgrade your 5.0 to 5.5R2 and then export objects (users) you want to import to 5.5R2.
You can also use the Import toolkit instead of upgrading to import data into your R2 database from another one of different version
Last Kentico CMS Import Toolkit for 5.5 R2Best regards,
Michal Legen