https://www.youtube.com/watch?v=YxXO_dj5Fw0&t=148s
Looks like you have to do a realtime recording from your iPhone to do this. THAT would be a huge PITA
The same method would apply in Reaper I think
I can't find what audio format the loops are anywhere. You could try getting to the FOLDER in your iPhone that holds those audio loops and drag and drop from there. It may or may not work.
Also, in Reaper under your Prefs/AudioDevice you will want to check the box:
Allow use of different input and output devices (Legacy option, not recommended)
Set your INPUT device as the iPhone and your OUTPUT device as your audio interface going to your speaker if you are using that and monitors.
**** I cant test this here as I have no iPhone. Just TWO Macs here.