I used a test site for this :
A. Export e.g. 3 fields (The Id field and 2 more fields) from Sand database which contains more then 3 fields .
B. Import the above .csv to Live database.
The Import is ok but the values off the rest fields of the Live database are erased.
I do not think that this should be the proper behavior when Importing fields to Live Db
The proper behavior would be to add the imported fields to the live database, without zero-ing the rest fields.