Concatenate Data from different row from same table
You can use for xml to concatenate values from multiple rows. There are many other ways to concatenate rows as outlined here 

For XML example with your data above, first we create and load sample table

    CREATE TABLE [pecomment]([PE_CASE]    int,
                    [PE_ITEM]    varchar(4),
                    [PE_HIS_SEQ] int,
                    [PE_COMMENT] varchar(20));

INSERT INTO [pecomment]
VALUES
      (200, 0001, 9989, 'Comment 1'),
      (200, 0001, 9990, 'Comment 2'),
      (200, 0001, NULL, 'Comment 3'),
      (200, 0002, 9986, 'Comment 4'),
      (200, 0002, 9987, 'Comment 5'),
      (200, 0002, NULL, 'Comment 6'),
      (201, 0001, 9926, 'Comment 7'),
      (201, 0001, 9937, 'Comment 8'),
      (201, 0001, NULL, 'Comment 9'),
      (201, 0002, 9933, 'Comment 10'),
      (201, 0002, 9921, 'Comment 11'),
      (201, 0002, NULL, 'Comment 12');


Then use for xml to concatenate the comments, grouping by PE_CASE and PE_ITEM

SELECT [PE_CASE],
      [PE_ITEM],
      NULL,
      [PE_COMMENT] = SUBSTRING((
      SELECT '+ '+[PE_COMMENT]
      FROM [pecomment] AS [t2]
      WHERE [t1].[PE_CASE] = [t2].[PE_CASE]
           AND [t1].[PE_ITEM] = [t2].[PE_ITEM]
      ORDER BY [PE_CASE],
             [PE_ITEM]
      FOR XML PATH('')
      ), 3, 1000)
FROM [pecomment] AS [t1]
GROUP BY [PE_CASE],
        [PE_ITEM];


This should give you what I think is your example desired result, hence the null in PE_HIS_SEQ.