I guess it’s possible but all I can see is gear to plug in to a motorbike,
instead of my night light battery.

I know Wxposure do it & I remember there was a light unit with it built in, but is there a cheap connector widgit.

No. I made one myself, but it was the wrong kind of voltage regulator and it got really hot. There are better kinds.

But by far the cheapest and easiest way for long bike rides is to buy a ‘portable charger’ from Amazon, they are cheap as chips now. Tape to bars, done. You can get a load of charge in half an hour from a battery, they charge quickly.

The dx waterproof battery case has a usb socket

http://www.dx.com/p/pannovo-b-c18-waterproof-4-18650-battery-pack-case-for-bike-light-cell-phones-black-355547#.WBzJT4ynxpU

As above, I’ve used one of those battery boxes for several audax’s where I needed to charge the Garmin and my phone.

Some of the Moon lights have a USB outlet from the battery, eg this one.

lupine USB one