Both Gunther an David are ...

Both Gunther an David are correct. Permeate correction for hardness can be achieved in a number of ways, including those mentioned below plus dosing using NaOH and Calcium Chloride if RO permeate is low pH. The best method will depend on the daily permeate capacity of the system, along with the pH of the raw water and RO permeate. I assume the treated water TDS requirement is very low given the selection of RO instead of Nano membranes. For smaller flow systems, Calcite media filters work well and are more economical initially over lime and CO2 based systems. Calcite media dissolves at low pH until pH is neutralised, adding calcium and alkalinity to the water. Permeate blending will work only if the product water TDS requirement is higher, but in this case I would consider Nano membranes.