Pre-open is just a one tick value that gets updated on the terminal at 9:08 AM. And most importantly pre-open value is nothing but the actual open value of the stock.
Take for example: 23-5-18 - In Pre- open - TATA MOTORS was the top gainer in Nifty. check the pre-open price , its 311.10
Now lets look at TATA MOTORS value - the open value is same 311.10
So to download the historical pre-open value , you just have to download the EOD data directly from NSE historical data and use the open value of the stock, which is nothing but the pre-open value.