Can I use our existing certificate authority instead of yours?

Not today. Passbeam provisions and runs a certificate authority per client, and there is no import path for an external one — a certificate has to chain to the authority Passbeam issued for that client, or it is refused.

For most people that is the point rather than a restriction: not running a CA is half of what this replaces, and the per-client authority is what stops one customer's certificates meaning anything on another customer's network.

It is a fair question if you already have a PKI you trust and a reason to keep it, and it is on the list to look at properly rather than dismissed. If that is you, say so — it changes the priority, and knowing what you need it to do is worth more to us than a guess. The half that would take work is issuance; trusting an external root is the smaller part.