Traffic signals will automatically get imported if the map region(map.osm) that you are exporting from openstreetmaps contains traffic signal information.
When you download osm maps ensure that traffic signal information is available for the region that you download as shown below:
After importing map.osm and then creating your network scenario, when you open the configuration in SUMO it will initially look as shown below:
To confirm whether traffic signals are imported, you can go to the settings icon which is highlighted in the image shown below:
Following this go to the junction settings under the junctions tab and select "by type" option from the Color drop-down list as shown below:
Now you will be able to see traffic signals highlighted as per the default color that is listed in the above properties as shown in the image below:
Upon simulating the network in SUMO or using NetSim-SUMO interface, you will be able to find that vehicles halt whenever signal turns red at the junctions.
Users can manually add traffic signals over the open street map (map.osm) region that they have downloaded, using tools such as JOSM. Following is a simple example of how this could be done https://www.youtube.com/watch?v=lFcJM5_bijU