It is currently not possible to make a 3-phase bridge to feed Source 2s from a 1-phase fuxe box.
Can I prepare a 1-phase installation for a 3-phase expansion?
No.
What should I do?
You should feed the charger over 1 phase, just L1+N, and nothing else. After the fuse box has been upgraded to 3-phase, someone should come back to the installation to connect L2 and L3. The charger should then be re-configured with the Installer App (which can be done remotely).