How can I configure mosquitto in Pi image to work as broker

Hi - I am using the SD card Pi image. I see that mosquitto is installed. Is it possible to use it as a broker to allow me to send and monitor configuration data on my device using mqtt?
So emonhub / emoncms will not consume the data. I want to send the data from a phone app via an mqtt broker running on the emon pi to my device and then get a reply. I guess what I really want to know, is this possible by just configuring hub or will it require that I write my own code to do this?

emoncms only consumes data on the emon base topic. Data published/subscribed to on any other topic does not get consumed by emoncms.

I think you need to understand how MQTT works a bit more.

Am I correct in assuming that if the emoncms data coming in via mqtt requires a user name and password that this will also apply to any other use of the broker? Or are these credentials just used by emon?

It is nothing to do with data, the username/password are used by the client connecting to the broker. You need to read that link again so you understand how MQTT works.