Can i wire a pump with 2 Voltages like this?
It's unlikely that your step down would care about not having a load (although we'll need the part number to verify this).

What may be an issue is that when the pump is spinning, the moment you switch it to 8V, it may try to push too much current into the step-down. I would still say this is fairly unlikely, and since this is pretty unlikely to damage your pump or your power supply, the only risk is your $4 step down, which isn't a huge risk. 

The actual chip is that big black thing next to the copper coil, below the display. That will have a series of numbers and letters on it. If you post those numbers/letters here, the answers will be much more specific and helpful.