Append existing lead record data

Append existing lead record data

Hi..I'm looking to append all existing lead record data with a new set of information I need. When I use Import, I get the Overwrite data, but I dont want to overwrite the data since there many new fields updated on them since importing to zoho.

To simplify this, here's the real-world scenario:

1. I imported over 3000 leads to CRM
2. The leads were worked on by the sales team with new data added to them
3. Now, I've just got a list of all their twitter handles. I have created a custom field for leads called Twitter handle. Now I want to append each lead record with this data.

How do I do this?