I don't think so, given that the V plus box remains the property of Virgin Media. They might be seriously upset if you gutted their stb to put in a new hard drive. and the operating system might not support a larger hard drive anyway.
However, you are supposed to be able to download recorded programmes to a video recorder, although people have had various problems, including losing the sound track of the recorded programme. Use the search facility and you'll see what I mean.