pH of RO/DI water

[Aquattro]

No, you're adding water to make up for the evaporation. The minerals of the evap are still there and will combine with the fresh, leaving it in a state as before the evap. You don't need to do anything.
The reason it's 6 is that it has no carbonates in it (alk ) and any slight uptake of CO2 can affect the pH a lot. Once it's added to the bigger solution containing carbonates, the buffering capacity will return and you're good to go.