Currently, with Assets, when you perform a CSV Import to update some attribute values from an Object, the existing data will get overwritten by the import.
For example, let's say you have the following Object types:
User (attribute Name)
Product (attribute ProductName)
Now consider having a User Object which already has a Product "Jira".

If you do a CSV Import to add more products to this object, then you need to add the existing value for that attribute in the CSV:

Name, Product
John Smith,Jira||Confluence||JSM

Otherwise, the data will be overwritten by the new CSV values

Name, Product
John Smith,Confluence||JSM