Am I able to use PBMDB to seed a second node before setting up a replicaset?

Hello,

Are we able to use Percona Backup for MongoDB to copy a current stand alone node to a second server that will act as a secondary node and ultimately be running as a “hidden” replicaset node. Essentially, I would like to see if I can use this software to “seed” the second node without using something like rsync.

We are using Percona Server for MongoDB 7 in a Docker container with great success. We have around 5TB of storage.

Also, if we were able to use this tool for syncing, can we do it while the database is live and serving requests?

Hello, this is not possible currently with Percona Backup but please vote the FR here Jira in order for us to prioritize it.
For now, you should add the new node and let the MongoDB initial sync process copy the data automatically.