I'm not sure this can be done directly, as Apple has created this mode in order to save space and not to leave Originals on the interested device in order to take a quick look at those even when Offline.
But there's a way around you may find very useful and suitable for your specific case, and not only.
iXpand flash drive
Earlier this year, San Disk and other brands have released a new type of flash drives, this flash drives are specifically designed to have a USB connector and a Lightning one.
This allows you to connect it directly either to your Computer or to your iDevice.
This way you can save all your important files, photos and videos in it and access them Offline, even while flying, via your iDevice, simply connecting the flash drive to it.
The San Disk flash drive is called iXpand, and it comes in different storage capacities, from 16GB up to 128GB.
Of course you're not restricted to purchase a San Disk one, even because I'm not promoting them in any way, but their flash drive's been the first one I've personally heard of.
Although, if you're interested to take a closer look at it, you can find further more details on their dedicated page here.
Otherwise, you can check out this 9to5mac article which features some of the best new lighting flash drives for iDevices.