You could use toString() method but that won't actually work well because of character encoding rules. You may need to manually convert the byte array to a string format that HSQL understands then just use a setString on a prepared statement to set the data.
Although not all jdbc drivers support this method of creating records with blobs/clobs. Others require multiple passes to do an insert. The first to insert a record without the clob/blob, the second to read the record and obtain a pointer to the clob/blob, and a third to actually set the clob/blob data. This is why you cannot create a clob/blob java object yourself, you can only use one the system gives you access to.