Yes, you can board the train at Shin-Imamiya. Since all seats on the Rapi:t are reserved you will need to make sure that you board the correct train.
Your ticket will have the departure time from Namba, you can track when this train will arrive at Shin-Imamiya on Google maps.
Alternatively, you can buy a ticket at a ticket machine or ticket office at Shin-Imamiya station.