postgresql – Creating a Postgres 11 cluster on a production server without downtime

I have a production server that runs Postgres 11. The database is> 10 GB. We use pgBouncer.
Is it possible to upgrade to create a slave (and a cluster of course) without a downtime?

Currently we have only one instance of Postgres 11 and pgBouncer.
What we would like to achieve is
1. Create a slave and transfer read requests to it
2. Probably pass master / slave because the current database runs on a "bad" server
3. Do other cool things with cluster

The problem here is that