When you import customers to X-Cart pay attention to each Wizard's page. Import is a rather complicated procedure and it may be destructive if performed incorrectly. Create database backup before you import customers to X-Cart especially if you do it first time.
Most frequent import errors are date, time and number conversion issues, caused by improper format settings (specified at previous page) or improperly assigned column (i.e. if you assign column containing category ID to parent_category database field). Date/time conversion errors might be ignored whereas other errors like this one (Could not find image:image_name.gif) better to be fixed.