Do you mean Open Street maps? If so, then yes, they can be put onto an SD card and the card will then go into the device. They will be fully routable, too.
I assume Ordnance Survey maps can be, too. Whether or not they are routable on the device, I have no idea. Why not just try?
PS There is a dedicated GPS section. Try it, it’s pretty good. You’ll get a much wider audience, than the narrow world here.