NOAA Radar Pro app allows you to add multiple locations in a variety of ways.
You can add a location if you simply tap the screen in necessary place - this will drop a blue pin on it and then tap on the star icon in the bottom right corner to add the location to your list:
Also, you can tap on the map at the place you need. Hold the location’s name and swipe up to see the detailed weather screen. Tap the star at the top right corner to bookmark the location:
Alternatively, you might consider adding the location directly to the list.
For this - tap "Locations List" button in the bottom right corner of the app's main screen and then, on the next screen, tap + button:
On the next screen - enter the location's name in the search bar. After this - tap one of the suggested variants in order to add it to the list. This will also drop a purple pin on the map.