I have a csv file with the following fields:

n --> order number (not important)
firstname --> first name (obligatory)
lastname --> last name (obligatory)
email --> e-mail address (obligatory)
tipo --> attribute I want to to use to filter names for a specific survey.

When I import to the central participant database, I get only the obligatory fields above. I've already tried to create "tipo" attribute previously to importation, but didn't work.

What I did, step-by-step:

1- Participant panel: click the import .csv button;
2- Next screen: choose the file to import, with character configuration and separator set to automatic;
3- Next screen: shows up three areas. On the left, shows the attributes fields found in csv file. The central area is blank, but a text asks to place a csv field in order to create a new one. I don't know how it works, since it's impossible to type anything ther. So, I drag the attributes of the left area into it. OK. But, when I click the Continue button, nothing happens.
4- In the right area thers is a white rectangle with the name ao an atribute, if I created it prevoiusly. I can drag the existing attributes into it and click Continue. Works, but the values os the csv file columns that are non-obligatory doesn't come to the participants table.

Sorry for the long text, but is there any solution I can use to import non-obligatory fields to central database?

An observation: the database is not empty. So, new participants are to be added.