Hi,
I think the DBMS_LOB.WRITEAPPEND procedure might help you.
Code:
DBMS_LOB.WRITEAPPEND (
lob_loc IN OUT NOCOPY CLOB CHARACTER SET ANY_CS,
amount  IN BINARY_INTEGER,
buffer  IN VARCHAR2 CHARACTER SET lob_loc%CHARSET);

lob_loc - Locator for the internal LOB to be written to.
amount  - Number of bytes (for BLOBs) or characters (for CLOBs) to write, or number that were written.
buffer  - Input buffer for the write.
Regards,