This Oracle tutorial explains how to use the Oracle/PLSQL TO_LOB function.
The Oracle/PLSQL TO_LOB function converts LONG or LONG RAW values to LOB values.
The syntax for the Oracle/PLSQL TO_LOB function is:
TO_LOB( long_column )
long_column can be a LONG or LONG RAW value.
insert into companies (lob_column) select TO_LOB(long_column) from emp;