Is it possible to transfrom a transfer function into voltage?
As your controller must be implemented digitally in discrete time, you have two options : 


Design your PI controller in the continuous time (s) domain and
convert to discrete time.
Convert your plant to discrete time and design the PI controller
accordingly.


Method (2) is generally preferred because it handles delays more easily and the controller is directly implementable without additional transforms. Look at this answer for more info.

Once you have the controller in discrete time make sure it's in polynomial form:
$$\dfrac{Y(z^{-1})}{X(z^{-1})}=\dfrac{b_0+b_1z^{-1}+..+b_kz^{-k}}{1+a_1z^{-1}+..+a_kz^{-k}}$$

and convert to a difference equation that can be directly implemented in hardware:

$$b_0x(n)+b_1x(n-1)+..+b_kx(n-k)=y(n)+a_1y(n-1)+a_2y(n-2)+..a_ky(n-k)$$