Hello everyone, Anybody knows if fields that were added to the form through N/ui/serverWidget Module (form.addFields.......) via userevent beforeload script can be shown in CSV mapping import?

nope, since it will only be displayed on the UI side. If you want the field then might request the record to be saved on an actual field (custom field or core fields)

Do you think it is possible to add a custom subList for journal entry via UI custom fields which can be mapped in CSV import?

instead of using ui/serverWidget why not just add the subtab and field just set the permission level so that specific role can only have access