Can a custom label be applied to a contact field using partial text from the field value?
Applescript to Create Contacts:

    set phoneDad to "Dad Mobile" --Use your own custom variables imported from CSV
-- would need to parse CSV

    tell application "Contacts"

        set thePerson to make new person with properties ¬
            {first name:"John", last name:"Doe", organization:"ABC Apps"} ¬


        -- see the "Contacts" AppleScript dictionary
        -- for other attributes than may be added

        make new email at end of emails of thePerson with properties ¬
            {label:"Work", value:"john@example.com"}
        make new phone at end of phones of thePerson with properties ¬
            {label:phoneDad, value:"555.555.1212"} --Use a variable to replace hardcoded number
        make new url at end of urls of thePerson with properties ¬
            {label:"Work", value:"http://www.example.com/"}     
        save

    end tell


You can set custom variables based on your CSV parsing.

This should point you in the right direction.  So, in answer to your question, with Applescript you can add custom fields.  Just use a similar syntax to the above code.  You just need to parse your CSV files accordingly.  I don't know what 3rd party app you use, but this is routinely done directly from database apps such as Filemaker.



Further reading: vCard Wikipedia


  vCard 3.0
  
  BEGIN:VCARD VERSION:3.0 N:Gump;Forrest;;Mr. FN:Forrest Gump ORG:Bubba
  Gump Shrimp Co. TITLE:Shrimp Man
  PHOTO;VALUE=URL;TYPE=GIF:http://www.example.com/dir_photos/my_photo.gif
  TEL;TYPE=WORK,VOICE:(111) 555-1212 TEL;TYPE=HOME,VOICE:(404) 555-1212
  ADR;TYPE=WORK:;;100 Waters Edge;Baytown;LA;30314;United States of
  America LABEL;TYPE=WORK:100 Waters Edge\nBaytown\, LA 30314\nUnited
  States of Ameri  ca ADR;TYPE=HOME:;;42 Plantation
  St.;Baytown;LA;30314;United States of America LABEL;TYPE=HOME:42
  Plantation St.\nBaytown\, LA 30314\nUnited States of Ame  rica
  EMAIL;TYPE=PREF,INTERNET:forrestgump@example.com
  REV:2008-04-24T19:52:43Z END:VCARD