When am trying to update contacts using Dataloader then multiselect picklist values for existing records gets overridden.I want values to be added to current multiselect picklist without overriding values with what I have on the existing record.Is there a way to do this?
You need to create 2 set of csv where the first csv contain those contacts where you want to override picklist values.
And in second csv if the picklist column is still there in the csv file, you will not map this column during update.
Salesforce dataloader doesn't have a provision to decide dynamically which values to be overridden. It just updates ALL values which are getting mapped.