I am afraid I don't fully understand what you are asking. I have provided you two ways to ensure both your raw and imported files are saved in a single segment, and therefore can be read and exported as such. Do you mean that Brainstorm sometimes processes a data file by blocks? This is to avoid memory issues for files that are so big they would not fit in memory. You can increase your block size in your Brainstorm preferences (File -> Edit preferences). By default, it is 100 megabytes.
Sorry if I again misunderstood you, perhaps include a screen capture to clarify what you are asking?
Martin