First your class must implement java.io.Serializable.
MultipartEntity e = new MultipartEntity();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeObject(yourserializableobject);
oos.flush();
oos.close();
InputStreamBody isb = new InputStreamBody(new ByteArrayInputStream(baos.toByteArray()), "o");
e.addPart("o", isb);