Not easily,
Allowing Roon to be used as a ‘pseudo’ audio device that you can stream too, for example via AirPlay, has been suggested a few times.
There are existing 3rd party Roon extensions that support the above functionality, but they are tricky to setup and far from perfect. Very much in the tinkering category, whether it’s a viable option will largely depending how technical you are and your willingness to tinker.
You don’t mention how you currently stream from Roon to your amplifier, is your Core connected directly to your Amp? What mobile device to you use, iOS or Android?
The reason I ask is that if your amp or streamer supports AirPlay (or Chromecast Audio) you could just bypass Roon completely and stream direct to that device. But I’m guessing that isn’t an option?
The other option is to get a cheap streamer like a WiiM and use it purely for the purpose of streaming from your phone outside Roon.