Trellis: add a user after deployment (can't provision anymore)

I need to set up SFTP accounts to the staging server. I don’t want to reset the whole setup, just add a few users that will connect via sftp.

What is the proper way to do that? By ssh-ing into the Staging VM, or can I do it via Trellis, but without erasing the current staging?

Thank you very much for your insight.