Include a schematic of your situation. You can create/design/construct a junction if that is what you want to do. This can be added to the model by connecting one reach to another in the Geometric Data Editor. I suggest that you practice entering it into HECRAS by starting a new project and create a single reach. Then create a second reach that ties in the middle of the first. HECRAS will prompt you for the data/naming of the junction. Goodluck & Have fun!