Not what you’re hoping to hear, but …
The User export file contains fields from the User table.
The “days_visited” field is from the User_Stats table.
It would be “anti-normalization” to have the same identical data more than once (eg. if the field were to be in both tables). IIRC what you see are the results of a JOIN query that does use the User table, but also other tables related to a user.
That said, with the Data Explorer plugin you should have no problem running such a JOIN query that generates an export file with that field included.