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.
As @santanu clearly pointed out, the practical way would be to first get the export of the picklist column alongwith ID column (retrieve the picklist defined in global values set in salesforce). Then create a second CSV file and copy all the picklist coulmns to that file, add the existing values, separated by semi colon and do update operation via data loader. It'll update both the old & new values altogether.
The first exported file, you can use it as a reference for comparison.