Andreas,

When importing, the mapped field values are replaced so you must add the new values to the existing in your imported source.  You can export the current values (Reports - Inventory - Export) then update this list by adding the new values to the current value when present.  You will notice that tag values are terminated with semi-colons so if you want to add more than one tag, be certain to include a semi-colon.  Also, if you include a tag that is not currently in the list shown on the Inventory Tag Maintenance window, it is added as part of the import.