Assuming that you're referring to the MiXplorer app (cannot find any reference online to a 'MiX Explorer' file manager Android app), it already has full read/write capability as is. With Android, you phone's internal storage is set up using the ext4 file system, and any microSD card will need to be either FAT32 or exFAT (or ext4 of course but that's a conditional matter as microSD cards are by default using one of those two FAT variants).
Anyway, please specify by what you mean by a USB file system. Is it an external hard drive? A webcam? Another smartphone?
Or is it matter not being able to find this mystery USB device in MiXplorer's menubar?
A strong point for MiXplorer is it includes a lot of options to configure and optimize not just features but the entire user interface to suit your own particular preferences. But a common user interface includes a hamburger icon (three horizontal lines) in the upper menubar. If you tap on that you should see options to change the view window to a different volume (i.e. internal storage to toggle between a mounted USB drive or to a microSD card).