If you need to do it from SQL then you can use procedure:
CREATE FUNCTION get_text(aHeaderID int)
RETURNING lvarchar;
DEFINE result lvarchar;
DEFINE vcfld lvarchar;
LET result=NULL;
EXECUTE PROCEDURE IFX_ALLOW_NEWLINE('T');
FOREACH cur1
FOR SELECT Descr INTO vcfld FROM OldTable WHERE HeaderID = aHeaderID ORDER BY LineNum
IF result IS NULL THEN
LET result = vcfld;
ELSE
LET result = result || '
' || vcfld;
END IF;
END FOREACH;
RETURN result;
END FUNCTION;
(notice usage of IFX_ALLOW_NEWLINE
and line breaking when updating result)
Then you can fill NewTable
using:
UPDATE NewTable SET Descr=get_text(HeaderID);