Hello NLund,
There are a few ways to do this. It would help if you described the source of the additional flow, but I will tell you based on the assumption that you have a model of a river working with an existing steady flow file, and there is a a channel or pipe which you are not modeling, which is discharging into the river.
1. Open the Steady Flow file: >Edit>Steady Flow Data…>
2. Find the appropriate River and Reach containing the cross section where you want the additional flow to begin. It is extremely important that you understand how and where the Flow Data changes are taking place both upstream and downstream of the point Q. Once you understand this, you will need to either “Add A Flow Change Location” at the desired cross section, or modify it if it exists already. Then, you will need to add the additional point Q to the existing Q in that cross section and also to every flow change location downstream of that point in the model. In other words, HEC-RAS does not have the option to simply add a point Q – you must specify the cumulative flow at every flow change location in the model.
Hope this helps.