Custom field (and function) in Import

I have a custom field "source_email" which store the seller(s) contact email(s).

What I would like to do is when I import (an already existing) product and this field is given, but differs from the existing one, then append it. So dont update any informations except add a new value to the source_email field. It is possible somehow? Maybe in this way not, if not do you have a workaround idea?

e.g.: in DB
"product_code", "product_description", "product_quantity", "source_email", ...
"H1839", "This product is amazing....", "4", "hergo@propeller.org", ...

when I import this
"product_code", "product_description", "product_quantity", "source_email", ...
"H1839", "This is new desc but....", "2", "somehow@whiskey.info", ...

would be the result
"product_code", "product_description", "product_quantity", "source_email", ...
"H1839", "This product is amazing....", "4", "hergo@propeller.org, somehow@whiskey.info", ...

My first idea was use the product_related, or product_parent_id fields.. but I rather ask before do anything wrong..

The import system replaces the product information. It is not made to add information to already existing product information. You will have the same issue with the related column or other columns.