Digital to Analog Conversion with STM32F7 Discovery Board
Do I need an extra board to perform DAC with STM32F7 DISCOVERY board


Yes, PA4 and PA5 are not accessible (which is weird that they would not make them accessible )


  What should I do to access the PA4 / PA5 pins for DAC operations that
  are essentially required in closed-loop control projects?


The DAC probably isn't the best for control as it is only 12-bit and 3.3V. The STM32F7NG also doesn't have a lot of 0.1" headers, it is more of a minicomputer board. You could find PA4/PA5 on the board and hack into them, at the same time it would probably disable the high speed USB because those lines need to be impedance matched. 

I would suggest a 16-bit spi dac: https://www.google.com/search?client=firefox-b-1-d&amp;q=sparkfun+16bit+dac