AC to DC conversion using without neutral
No, it's impossible. You need a phase and a neutral for anything using electricity, including step-down converters.

What you can do, is to look for distribution boxes and find where the wires are going. Behind your old wall switches, there are at least two wires attached. If there are more than 2, then it's more complicated because you have to deal with go-and-return switches where two switches command the same appliance. So for the sake of simplicity, I will talk about the case when there is only one switch for one appliance and two wires on this switch.

If you can find and disconnect these two wires at the connection box and make 100% sure they are not connected to anything else, then you can reconnect them, one to neutral, the other to a phase.
Then, remains the question about the wires going from the smart switches to the appliance. If these wires don't exist, in any case you will have to add extra wires to complete the circuit. At this occasion, you can add proper wiring for the AC-DC supply + a Earth wire (yellow-green).

I advise you to do that with a friend or an acquaintance who knows a little bit about electrical installations and can help you on site. ;)