POI files are simply advanced GPX files so if you place them into the GPX folder that should work. Or if you have a POI loader it will load them directly to the device.
Sadly I tried placing it in the GPX folder but coudln't find any custom POIs even though one was for my house which was ground zero.
I'm using GSAK, is there any reason why that wouldn't work with a Montana?
I tried the Garmin POI loader but got an error when I tried to run it. I'll have to try that again and see if I can figure out why it generates an error.