I’m reading that as the original application is not using Akka persistence at all, and maybe not event sourcing at all.
I’d recommend storing the initial state/balance as a first initial event for each persistent actor. That could even be done by a migration job and not being part of the real EventSourcedBehavior that is later used in real production.
One difficulty might be if the legacy application is still running and changing the data while the migration job is running or afterwards.
Also, as Johan hinted, I would recommend using EventSourcedBehavior in Akka 2.6 rather than the classic PersistentActor if you are building a new application.