You can append a contact's existing value(s) for a multiple checkbox property with the additional value(s) in your import by using a semicolon in your CSV.
- If you were importing new values to add to an existing value in a contact record, the cell should look like this where you place a semicolon first and in between each value:
;value 2;value 3;value 4
- Or if you were importing a completely new set of values, the cell should look like this, with semicolons in between each value:
value 1;value 2;value 3