There are a couple of ways to do this. You might try importing this field into a different field and then running a script that updates your field after you import.
Your script could
find all records where this new field is not empty
Loop through the records doing the following on each:
Use IsEmpty ( filter values ( current field, import field ) ) to determine that the imported value needs to be added
If so, use Set Field [table::valuelistfield ; table::Valuelistfield & ¶ & table::import field ) to append this value to the current list of values in the field