Download imagery form Google or other server map(with Mobile Atlas Creator or other apps)
Convert it to JNX maps with a JNX tools like mapc2mapc, mobat2jnx, map2jnx
But its no possible to open a JNX map in BaseCamp directly. Instead, you need to copy the jnx map to your device first (or, alternatively, to a USB flash)
After the device is plugged, open BaseCamp and your JNX maps will be shown in the device.