im well aware of the usual process, i ran my own body shop for 5 years..., but you can add it before hand to ensure the colour to thinner mix is the same throughout the whole process to avoid colour mix issues. you can actually buy pre mixed paint in standard colours like diamond white, that just need hardener added..

thinner should be added to the paint to acheive the correct viscosity, it doesnt matter if its added before or after, as long as the viscosity is correct.

you dont have to be a rocket scientist to calculate how much thinner to add, so when a set ammount of hardener is added the viscosity is correct,