Ransom Brokers are brokers after all, not just ransom buyers. It would make sense that you should be able to talk to them to pay soldiers' ransom in exchange for them joining your party. The available troops could be some combination randomly generated troops (including bandits and minor clan troops) and troops that have been ransomed recently. That said, the availability of these troops ought to be kind of limited in quantity, since you'd expect that since these brokers are 100% ethical, their families would be given priority on ransom buy outs, so wouldn't be available to any random lord/party leader that walks into the tavern.
We can just pretend this is also how AI lords get their respawn armies which somehow always come with a few high level troops.