Automatically choose between bid/ask price at position opening/close

[uelel]

I am sorry if this problem has been already discussed, I just did not found anything related.

I want to include real-time spreads during my backtesting with backtrader.
For that purpose I have M1 data with bid prices and spreads for each candle.
My idea is simple: to open long positions and close short positions with ask prices.
To implement this feature the data feed has to be accessible by whatever class that actually opens/closes positions.

Do you have any idea how to implement this feature?
What class is responsible for setting price during position opening/closing?