Hypothetically could I bring a portable speaker and use my phone to get the radio signal and play it through the speaker? My truck does not have a radio.

Yes. If your phone picks up FM radio signals

Be aware that almost every phone requires headphones to listen to the fm radio as it uses the cable as an antenna. It may work with an aux cable but I am not sure.

No, there is no app that ita available to play it through a phone

If you have an app or phone that receives FM radio. You can connect to a speaker via Bluetooth or cord and play it just like you would music.