How to Handle Adding 2nd Node to single MariaDB Galera "cluster" already in production?
The best way would be to use the xtrabackup-v2 SST method.  It is non-blocking for most of the transfer; there is a short blocking period at the end of the transfer when it has to get the latest data from the donor, but that is usually only a few seconds at most.  The other SST methods would need the donor to be offline for a much longer period since they aren't designed for online transfers.