DECLARE
  lob1 CLOB;

BEGIN

  SELECT e.xml_col.getClobVal() INTO lob1 FROM "XML"."XML_TABLE" e WHERE KEY_COL=3 ;
  DBMS_LOB.APPEND(lob1, 'is a test node</test>');

END;

22275, 00000, "invalid LOB locator specified"
// *Cause:  There are several causes:  (1) the LOB locator was never
//          initialized; (2) the locator is for a BFILE and the routine
//          expects a BLOB/CLOB/NCLOB locator; (3) the locator is for a
//          BLOB/CLOB/NCLOB and the routine expects a BFILE locator;
//          (4) trying to update the LOB in a trigger body -- LOBs in
//          trigger bodies are read only; (5) the locator is for a 
//          BFILE/BLOB and the routine expects a CLOB/NCLOB locator;
//          (6) the locator is for a CLOB/NCLOB and the routine expects 
//          a BFILE/BLOB locator;
// *Action: For (1), initialize the LOB locator by selecting into the locator
//          variable or by setting the LOB locator to empty.  For (2),(3),
//          (5) and (6)pass the correct type of locator into the routine.  
//          For (4), remove the trigger body code that updates the LOB value.

SQL> @v

Version Oracle : 9.2.0.8.0

SQL> declare 
  2    lob1 clob;
  3    val  xmltype := xmltype('<x>val</x>');
  4  begin
  5    lob1 := val.getClobVal();
  6    DBMS_LOB.APPEND(lob1, 'is a test node</test>');
  7  end;
  8  /

PL/SQL procedure successfully completed.

SQL> @v

Version Oracle : 10.2.0.4.0

SQL> declare 
  2    lob1 clob;
  3    val  xmltype := xmltype('<x>val</x>');
  4  begin
  5    lob1 := val.getClobVal();
  6    DBMS_LOB.APPEND(lob1, 'is a test node</test>');
  7  end;
  8  /

PL/SQL procedure successfully completed.

SQL> @v

Version Oracle : 11.2.0.1.0

SQL> declare 
  2    lob1 clob;
  3    val  xmltype := xmltype('<x>val</x>');
  4  begin
  5    lob1 := val.getClobVal();
  6    DBMS_LOB.APPEND(lob1, 'is a test node</test>');
  7  end;
  8  /

PL/SQL procedure successfully completed.

declare 
  lob1 clob;
  val  xmltype := xmltype('<x>val</x>');
begin
  dbms_lob.createtemporary (lob_loc=>lob1, cache=>true, dur=>dbms_lob.call);
  lob1 := val.getClobVal();
  DBMS_LOB.APPEND(lob1, 'is a test node</test>');
end;
/

declare 
      lob1 clob;
      val  xmltype := xmltype('<x>val</x>');
    begin
     dbms_lob.createtemporary (lob_loc=>lob1, cache=>true, dur=>dbms_lob.call);
     SELECT e.xml_col.getClobVal() INTO lob1 from XML.XML_TABLE e WHERE key_col = 3;
     DBMS_LOB.APPEND(lob1, 'is a test node</test>');
  end;
 /

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.1.0 - Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production

SQL> CREATE TABLE XML(key_col number primary key, xml_col XMLTYPE);

Table created.

SQL> INSERT INTO XML VALUES (0, XMLTYPE('<x>val</x>'));

1 row created.

SQL> declare
  2        lob1 clob;
  3      begin
  4       dbms_lob.createtemporary (lob_loc=>lob1, cache=>true, dur=>dbms_lob.call);
  5       SELECT e.xml_col.getClobVal() INTO lob1 from XML e WHERE key_col = 0;
  6       DBMS_LOB.APPEND(lob1, 'is a test node</test>');
  7    end;
  8   /
declare
*
ERROR at line 1:
ORA-22275: invalid LOB locator specified
ORA-06512: at "SYS.DBMS_LOB", line 347
ORA-06512: at line 6

SQL>