Using FreeUDFLib do the following

CREATE PROCEDURE ADDTOMYBLOB( MYVARCHAR VARCHAR(100))
AS
DECLARE VARIABLE MYTEMPVARCHAR VARCHAR(30000)


BEGIN
SELECT BLOBASPCHAR(MYBLOB)
FROM BLOBTABLE
INTO MYTEMPVARCHAR;
IF( MYTEMPVARCHAR IS NULL )THEN
MYTEMPVARCHAR = MYVARCHAR;
ELSE
MYTEMPVARCHAR = MYTEMPVARCHAR || MYVARCHAR;
UPDATE BLOBTABLE
SET MYBLOB = STRBLOB(MYTEMPVARCHAR);
END

Done and dusted.