There is no append option when you write to a QVD file. It will always have to be rewritten in full. That is why you have to do the appending in-memory and concatenate the old data with you new data. So you have the old data stored in a QVD and new data has to be read from another source and concatenated with the table that was read from the QVD.
You have to have some statements between the LOAD .... (qvd) and the Store. Which should look a bit like this: