Is it possible to add a solar tank upstream of an existing "conventional" DHW tank?

Yes, a solar storage tank can be installed upstream of a "conventional" water heater. 

This saves energy thanks to solar pre-heating, and the solar system can operate optimally without being disturbed by the heat input from the boiler's auxiliary heat supply (which is in the "conventional" boiler).

However, it should be pointed out that this is not ideal with regard to the size of the technical room (2 tanks instead of one).  

This connection must be made in the following order: 

• Empty the water heater of its water
• Disconnect the cold water supply to the water heater.
• Install the solar cylinder with its peripherals (solar station, roof lines, temperature sensor at the heat exchanger at the bottom of the solar cylinder, etc.).
• Connect the cold water supply to the solar storage tank.
• Connect the hot water outlet of the solar cylinder to the cold water inlet of the water heater. It is preferable to install a non-return valve on this hydraulic line to prevent the thermosiphon effect of circulating hot water from the water heater to the cooler water in the solar storage tank.

The solar storage tank will thus make it possible to accumulate solar heat and return it to the water heater located downstream of the hydraulic circuit each time hot water is drawn.