I want to buy a Google Pixel 3XL but it does not have a radio. Is it possible to install a FM radio application on it anyway?
You can install the FM app but you will not get any signal because there is no chip on the device allowing to receive FM radio signals.
Your only solution is to install apps that stream FM radios such as TuneIn Radio. It offers a list of local (based on countries) and international radios. You can find other apps in the Play store.
If your media player is able to stream online content, you use it to stream radios if they offer online streaming. Create a playlist with the URL of the radio streams, then play them as you would with music. I use VLC to stream radios.