Can i withdraw blood prior to the pregnancy in order have some in case of emergency?

Yes: You can do this, but it is usually not necessary. This is called autologous blood donation. In cases where there is anticipated blood loss like placenta accreta it may be a good idea. I have recommended it for patients who are jehovah's witness since they do not accept blood transfusions in general. Blood used for transfusion in the hospital is thoroughly tested and is safe to use if needed.

Not sure: Firstly, blood doesn't last that long - shelf life is a month or so, although certain blood components can last much longer. So, you would have to give blood on some regular basis, and one of the universal findings in pregnant women is already anemia, which you might be making worse. This is not my area of expertise, but i strongly advise talking to whoever you are seeing for medical advise.

Autologous blood: Yes. This is a regular practice before surgery. A phone call to your local blood bank might be in order.