OTG integration


Hello, I want to integrate a cryptomator folder, stored on a USB-Stick and plugged in as an OTG Stick in my android 7 mobile phone. Unfortunately I can copy the folder with the help of explorer software to the phone but can´t open or integrate the folder on the usb Stick directly in the cryptomator app. Does anybody knows a propper way to do this? Many thanks for your help and ideas says
Detlef from Germany