Append unlimited value field with feed import

I develop a Drupal web app using D8.
I am using Feed import to import or update users. However, I would like the feed import module to update a user and add another value to a field that has unlimited values.
But the challenge I have is when I add the feeds import and select update option it overwrites the existing value of the unlimited value field instead of adding it up to it.

I have users, each month user make a contribution and I update the user details with the month contribution he made added to an unlimited value field called field_contribution.