User attributes can be filled in by your users, either as a text field or a drop-down menu. However, if you need to attach data to user profiles, you can run a user attribute import.
First, get your data file together in accordance with these guidelines:
CSV format
A column with the heading "email" - make sure the user emails listed in this column are all entirely lowercase
Make sure there are no user emails that are listed more than once in your file
Only include additional columns for the attributes you're currently trying to import (be sure to match the name of the attribute you created)
For example, if I'm importing student ID #s, my CSV file would only have two columns: email, Student ID #
Next, you'll run your import by going to the Users tab -> select 'Import User Attributes' in the sub-menu list -> import your CSV file!