Usa una LEFT JOIN.

codice:
sql = "SELECT UNI.IDUNI, UNI.DEFIN, UNI.IDARC, " _
& "ARC.NOME, ARC.VERS, UNI.NUMFILE, " _
& "UNI.NUMIND, UNI.NUMARC, UNI.DATACRE, UNI.DATAAPR " _
& "FROM UNITA UNI LEFT JOIN ARCHIVI ARC ON (UNI.IDARC=ARC.IDARC) " _
& "ORDER BY UNI.DATACRE"