You could import the data into a different field with an auto enter calculation set on the original field to combine the two entries. Then enable auto-enter operations during import.
Storing such a list all in one field, however, is rarely the best database structure. Storing such a list in a related table where each item is stored in a different records almost always is the better approach. With that approach you may be able to import directly into the related table and not have to do this kind of data concatentation at all.