I think that the topic I created (It is solved now) is what you are looking for.
"You can't append to an existing file created with an ObjectOutputStream, at least not without effort. There is a trick somewhere about extending ObjectOutputStream and overriding the `writeStreamHeader() method so as not to write the stream header the second time (...)" - answered by EJP
Please have look for my topic here:
You can also check the link below
It is what you were looking for?