Current State:

When you're Updating Objects in JSM Assets Cloud via CSV Import, your import configuration should have a unique attribute as an identifier while importing the updated objects. If the file doesn't contain that unique attribute, new objects are created instead of simply skipping the whole process.
 

Proposed Future State:

Have a way to search for attribute values, and if that matches simply add the values on that object.

Alternative:

Since Assets import process can't handle this, you'll have to either:

• Export the CSV of your current objects and perform the required changes for import
• Build a custom configuration (like using Automation based on AQL) to find the relevant objects and then append the needed data on the objects' attributes.